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Filed Pursuant to Rule 424(b)(5)
Registration Statement Nos. 333-183686 and 333-183686-01

         Subject to Completion
Preliminary Prospectus Supplement dated May 13, 2015

The information in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Prospectus Supplement
(To Prospectus dated August 31, 2012)

$            

LOGO

Aon plc

% Senior Notes due 20
With a full and unconditional guarantee as to payment of
principal and interest by Aon Corporation

        Aon plc is offering $            aggregate principal amount of        % senior notes due 20            (the "Notes"). The Notes will mature on                        , 20 . Aon plc will pay interest on the Notes on each                        and            , commencing on                        , 2015. The Notes will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. Aon plc may redeem all of the Notes at any time, and some of the Notes from time to time, at the redemption prices set forth in this prospectus supplement under "Description of the Securities—Optional Redemption." Aon plc may also redeem all of the Notes at a redemption price equal to 100% of the principal amount of the Notes plus accrued and unpaid interest, if any, to the redemption date in the event of certain changes in respect of withholding taxes applicable to the Notes or the Guarantee, as described in this prospectus supplement under "Description of the Securities—Optional Tax Redemption."

        The Notes will be fully and unconditionally guaranteed (the "Guarantee" and, together with the Notes, the "Securities") by Aon Corporation ("Aon Delaware").

        The Notes will be Aon plc's general unsecured and unsubordinated obligations and will rank equally with each other and with all of Aon plc's other existing and future unsecured and unsubordinated obligations. The Notes will not have the benefit of all of the covenants applicable to certain of Aon plc's existing unsecured senior indebtedness. The Notes will be effectively subordinated to any secured indebtedness Aon plc may have or incur in the future to the extent of the value of the assets securing any such indebtedness. The Notes will be structurally subordinated to the indebtedness and all other obligations of Aon plc's subsidiaries (though you may have a direct claim as to Aon Delaware by virtue of its obligations with respect to the Guarantee).

        The Guarantee will be a general unsecured and unsubordinated obligation of Aon Delaware and will rank equally with all of Aon Delaware's other existing and future unsecured and unsubordinated obligations. The Guarantee will not have the benefit of all of the covenants applicable to certain of Aon Delaware's existing unsecured senior debt. The Guarantee will be effectively subordinated to any secured indebtedness Aon Delaware may have or incur in the future to the extent of the value of the assets securing any such indebtedness. The Guarantee will be structurally subordinated to the indebtedness and all other obligations of Aon Delaware's subsidiaries.

        We intend to apply to list the Notes on the New York Stock Exchange or another "recognised stock exchange" for purposes of Section 1005 of the U.K. Income Tax Act 2007.

        Investing in the Securities involves a high degree of risk. See "Risk Factors" beginning on page S-11 of this prospectus supplement.

        Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these Securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

 
  Per Note   Total  
Public offering price       % $    
Underwriting discount       % $    
Proceeds to us (before expenses)       % $    

        Interest on the Notes will accrue from May             , 2015.

        The underwriters expect to deliver the Securities for purchase on or about                        , 2015, which is the fifth business day following the date of this prospectus supplement, in book-entry form through the facilities of The Depository Trust Company and its participants, including Clearstream Banking, société anonyme, and Euroclear Bank S.A./N.V.



Joint Book-Running Managers

Morgan Stanley   Barclays   HSBC   J.P. Morgan

The date of this prospectus supplement is May                , 2015.


Table of Contents

TABLE OF CONTENTS

Prospectus Supplement

 
  Page

About This Prospectus Supplement

  S-1

Where You Can Find More Information

  S-1

Disclosure Regarding Forward-Looking Statements

  S-2

Summary

  S-4

Risk Factors

  S-11

Use of Proceeds

  S-14

Ratio of Earnings to Fixed Charges

  S-15

Capitalization

  S-16

Description of the Securities

  S-17

Certain United States Federal Income Tax Consequences

  S-24

Certain United Kingdom Tax Consequences

  S-27

European Union Savings Tax Directive

  S-28

Book-Entry, Delivery and Form

  S-29

Underwriting (Conflicts of Interest)

  S-33

Incorporation of Certain Documents by Reference

  S-37

Legal Matters

  S-37

Experts

  S-37

Prospectus

 
  Page

About This Prospectus

  3

Where You Can Find More Information

  4

Information Concerning Forward-Looking Statements

  5

Risk Factors

  6

The Company

  7

Use of Proceeds

  8

Ratios

  9

Description of Debt Securities and Guarantees

  10

Description of Preference Shares

  26

Description of Class A Ordinary Shares

  27

Description of the Share Purchase Contracts and the Share Purchase Units

  27

Plan of Distribution

  28

Validity of Securities

  30

Experts

  30

        Neither we nor the underwriters have authorized anyone to provide any information other than that which is contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or any free writing prospectus prepared by or on behalf of us or to which we have referred you. Neither we nor the underwriters take any responsibility for, or provide any assurance as to, the reliability of any other information that others may give you. No offer to sell these Securities is being made in any jurisdiction where the offer or sale is not permitted. The information contained in this prospectus supplement, the accompanying prospectus, any free writing prospectus or any document incorporated by reference is accurate as of the date of the document in which the information appears. Our business, financial condition, results of operations and prospects may have changed after any of such dates.


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ABOUT THIS PROSPECTUS SUPPLEMENT

        This document consists of two parts. The first part is this prospectus supplement, which describes the specific terms of this offering. The second part is the accompanying prospectus, which describes more general information, some of which may not apply to this offering. You should read both this prospectus supplement and the accompanying prospectus, together with the documents incorporated by reference and the additional information described below under the heading "Where You Can Find More Information."

        If the description of this offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information in this prospectus supplement.

        Any statement made in this prospectus supplement or in a document incorporated or deemed to be incorporated by reference in this prospectus supplement will be deemed to be modified or superseded for purposes of this prospectus supplement to the extent that a statement contained in this prospectus supplement or in any other subsequently filed document that is also incorporated or deemed to be incorporated by reference in this prospectus supplement or the accompanying prospectus modifies or supersedes that statement. Except as so modified or superseded, any statement so modified or superseded will not be deemed to constitute a part of this prospectus supplement. See "Incorporation of Certain Documents by Reference" in this prospectus supplement.

        In this prospectus supplement, we use the terms "Aon plc" or the "Issuer" to refer to Aon plc (not including its subsidiaries), and the terms "Aon," "we," "us" and "our" and similar terms to refer to Aon plc and its subsidiaries (including Aon Delaware), unless the context otherwise requires and except as otherwise described below. We use the terms "Aon Delaware" or the "Guarantor" to refer to Aon Corporation, our wholly-owned subsidiary and the guarantor of the Notes. In 2012, we reincorporated in the U.K. and completed the reorganization of the corporate structure of the group of companies controlled by Aon Delaware, Aon plc's predecessor as the ultimate holding company of the Aon group. In this prospectus supplement, we refer to this transaction as the "Redomestication." Any references in this prospectus supplement to "Aon," "we," "us" and "our" or any similar references relating to dates or periods before the Redomestication refer to Aon Delaware and its subsidiaries or, if the context so requires, Aon Delaware alone.


WHERE YOU CAN FIND MORE INFORMATION

        We are subject to the information reporting requirements of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). In accordance with the Exchange Act, we file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission (the "SEC"). Our SEC file number is 001-07933. You can read and copy this information at the following location of the SEC:

Public Reference Room
100 F Street, N.E.
Washington, D.C. 20549

        You can also obtain copies of these materials from this public reference room, at prescribed rates. Please call the SEC at 1-800-SEC-0330 for further information on its public reference room. The SEC also maintains a web site that contains reports, proxy statements and other information about issuers, including us, who file electronically with the SEC. The address of that site is www.sec.gov.

        This prospectus supplement and the accompanying prospectus, which form a part of the registration statement, do not contain all the information that is included in the registration statement. You will find additional information about us in the registration statement. Any statements made in this prospectus supplement, the accompanying prospectus or any documents incorporated by reference in this prospectus supplement or the accompanying prospectus concerning the provisions of legal

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documents are not necessarily complete and you should read the documents that are filed as exhibits to the registration statement or otherwise filed with the SEC for a more complete understanding of the document or matter.


DISCLOSURE REGARDING FORWARD-LOOKING STATEMENTS

        This prospectus supplement, the accompanying prospectus and any documents incorporated by reference into this prospectus supplement or the accompanying prospectus contain certain statements related to future results, or states our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. They use words such as "anticipate," "believe," "estimate," "expect," "forecast," "project," "intend," "plan," "potential," and other similar terms, and future or conditional tense verbs like "could," "may," "might," "should," "will" and "would." You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: market and industry conditions, including competitive and pricing trends; changes in our business strategies and methods of generating revenue; the development and performance of our services and products; changes in the composition or level of our revenues; our cost structure and the outcome of cost-saving or restructuring initiatives; the outcome of contingencies; dividend policy; the expected impact of acquisitions and dispositions; pension obligations; cash flow and liquidity; expected effective tax rate; future actions by regulators; and the impact of changes in accounting rules. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include:

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        Any or all of these forward-looking statements may turn out to be inaccurate, and there are no guarantees about our performance. The factors identified above are not exhaustive. We and our subsidiaries operate in a dynamic business environment in which new risks may emerge frequently. Accordingly, you should not place undue reliance on forward-looking statements, which speak only as of the dates on which they are made. We are under no obligation (and expressly disclaim any obligation) to update or alter any forward-looking statement that we may make from time to time, whether as a result of new information, future events or otherwise. Further information about factors that could materially affect us, including our results of operations and financial condition, is contained in the "Risk Factors" section in Part I, Item 1A of our Annual Report on Form 10-K for the year ended December 31, 2014, as filed with the SEC.

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SUMMARY

        This summary highlights certain information about Aon plc, Aon Delaware and the offering of the Securities. This summary does not contain all the information that may be important to you. You should carefully read this entire prospectus supplement, the accompanying prospectus and those documents incorporated by reference into this prospectus supplement and the accompanying prospectus, including the risk factors and the financial statements and related notes thereto, before making an investment decision.


Aon plc

        Our strategy is to be a preeminent professional service firm in the world, focused on the topics of risk and people. We are the leading global provider of risk management services, insurance and reinsurance brokerage, and human resource consulting and outsourcing, delivering distinctive client value via innovative and effective risk management and workforce productivity solutions. We serve clients through two operating segments, Risk Solutions and HR Solutions. Risk Solutions acts as an advisor and insurance and reinsurance broker, helping clients manage their risks via consultation, as well as negotiation and placement of insurance risk with insurance carriers through our global distribution network. HR Solutions partners with organizations to solve their most complex human capital and related financial challenges in the areas of health, retirement and talent. We are dedicated to improving business performance and our clients' employees' experience by designing, implementing, communicating and administering a wide range of human capital, retirement, investment management, health care, compensation and talent management strategies. As of December 31, 2014, we had approximately 69,000 employees and conducted our operations through various subsidiaries in more than 120 countries and sovereignties.

        Our principal executive offices are located at 8 Devonshire Square, London, England EC2M 4PL. Our telephone number is +44 20 7623 5500.


Aon Delaware

        Aon Delaware is a wholly-owned Delaware subsidiary of Aon plc. Prior to the Redomestication, Aon Delaware was the ultimate holding company for the Aon group. See "About this Prospectus Supplement" and "Where You Can Find More Information."

        Aon Delaware's principal executive offices are located at 200 East Randolph Street, Chicago, Illinois 60601, and its telephone number is (312) 381-1000.

 

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Offering Summary

        The following is a summary of some of the terms of this offering. For a more complete description of the terms of the Securities, please refer to "Description of the Securities" in this prospectus supplement and "Description of Debt Securities and Guarantees" in the accompanying prospectus.

Issuer   Aon plc.

Notes Offered

 

$            aggregate principal amount of      % senior notes due 20    .

Maturity Date

 

                        , 20    .

Interest Rate

 

The Notes will bear interest from and including May     , 2015 at the rate of      % per annum, payable semiannually in arrears.

Interest Payment Dates

 

Interest on the Notes will be payable in arrears on each                and                , commencing on                        , 2015.

Guarantor

 

Aon Delaware.

Guarantee

 

The Notes will be fully and unconditionally guaranteed by Aon Delaware.

Ranking of the Securities

 

The Notes will be unsecured obligations of Aon plc and will rank equally in right of payment with each other and with all of Aon plc's other existing and future unsecured and unsubordinated obligations. The Notes will be effectively subordinated to all of the existing and future secured indebtedness of Aon plc to the extent of the value of the assets securing any such indebtedness. As of March 31, 2015, Aon plc had no secured indebtedness for borrowed money and had approximately $11.1 billion of consolidated outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, other current and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. The Notes will be structurally subordinated to all of the existing and future secured and unsecured indebtedness and other liabilities of Aon plc's subsidiaries (though you may have a direct claim as to Aon Delaware by virtue of its obligations with respect to the Guarantee). As of March 31, 2015, Aon plc's subsidiaries (other than Aon Delaware) had approximately $1.6 billion of outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. These liabilities (which exclude liabilities of Aon Delaware) constitute approximately 23.1% of Aon plc's total consolidated liabilities (excluding liabilities of Aon Delaware).

 

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    The Guarantee will be an unsecured obligation of Aon Delaware and will rank equally in right of payment with all of Aon Delaware's other existing and future unsecured and unsubordinated obligations. The Guarantee will be effectively subordinated to all of the existing and future secured indebtedness of Aon Delaware to the extent of the value of the assets securing any such indebtedness. As of March 31, 2015, Aon Delaware had no secured indebtedness for borrowed money and had approximately $4.3 billion of consolidated outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. The Guarantee will be structurally subordinated to all of the existing and future secured and unsecured indebtedness and other liabilities of Aon Delaware's subsidiaries. As of March 31, 2015, Aon Delaware's subsidiaries had approximately $1.4 billion of outstanding indebtedness and other liabilities, including trade payables, pension and other post-employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities, constituting approximately 24.2% of Aon Delaware's total consolidated liabilities.

Optional Redemption

 

Aon plc may at its option redeem all of the Notes at any time and some of the Notes from time to time, at a redemption price equal to the greater of:

 

100% of the principal amount of the Notes being redeemed; and

 

the sum of the present values of the remaining scheduled payments of principal and interest thereon (not including any portion of such payments of interest accrued as of the redemption date), discounted to the date of redemption on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate (as defined under "Description of the Securities—Optional Redemption"), plus    basis points ( %),


 

 

plus, in either case, accrued and unpaid interest on the principal amount of the Notes being redeemed to but excluding the redemption date.

 

 

On or after                        , 20    , Aon plc may redeem any or all of the Notes at a redemption price equal to 100% of the principal amount of the Notes being redeemed, plus accrued and unpaid interest on the principal amount of the Notes being redeemed to but excluding the redemption date.

 

 

See "Description of the Securities—Optional Redemption."

 

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Additional Amounts   Subject to certain limited exceptions, Aon plc and Aon Delaware have agreed to pay additional amounts to the holders of Notes from time to time in the event any interest payment on the Notes or any payment made under the Guarantee is subject to withholding or deduction in respect of Taxes (as defined in "Description of the Securities—Payment of Additional Amounts").

Optional Tax Redemption

 

In the event of certain changes in respect of Taxes applicable to the Notes or the Guarantee, Aon plc may redeem the Notes in whole, but not in part, at any time, at a redemption price equal to 100% of their principal amount plus accrued and unpaid interest on the Notes, if any, to the redemption date. See "Description of the Securities—Optional Tax Redemption."

Covenants

 

The indenture includes certain requirements that must be met if Aon plc or Aon Delaware consolidates with or merges into, or transfers or leases its assets substantially as an entirety to, another entity or person.

Use of Proceeds

 

We intend to use the net proceeds of this offering for general corporate purposes which, among other things, may include funding acquisition opportunities, repurchases of our Class A Ordinary Shares and repayment of all or part of our 3.50% senior notes due September 2015 (the "2015 Notes"). As of March 31, 2015, $600 million of our 2015 Notes was outstanding. See "Use of Proceeds." Certain of the underwriters or their affiliates may hold the 2015 Notes and, accordingly, may receive proceeds from this offering. See "Underwriting (Conflicts of Interest)—Conflicts of Interest; Other Relationships."

Listing

 

The Notes are a new issue of securities with no established trading market. Aon plc intends to apply to list the Notes on the New York Stock Exchange ("NYSE") or another "recognised stock exchange" for purposes of Section 1005 of the U.K. Income Tax Act 2007.

Risk Factors

 

See "Risk Factors" beginning on page S-11 of this prospectus supplement for important information regarding us and an investment in the Securities.

Further Issuances

 

Aon plc may, from time to time, without the written consent of and without giving notice to holders of the Securities, create and issue additional notes having the same terms and conditions as the Notes in all respects (other than the issue date, public offering price, and to the extent applicable, first date of interest accrual and first interest payment date of such notes). Those additional notes will be consolidated with and form a single series with the previously outstanding Notes; provided that if the additional notes are not fungible with the Notes for U.S. federal income tax purposes, the additional notes will have a separate CUSIP number.

 

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Trustee   The Bank of New York Mellon Trust Company, N.A.

Governing Law

 

The Securities and the indenture will be governed by the laws of the State of New York.

 

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Selected Historical Financial Data

        The following table sets forth the selected historical consolidated financial and operating data for Aon. The selected consolidated financial and operating data as of and for the years ended December 31, 2014, 2013 and 2012 have been derived from Aon's audited consolidated financial statements and related notes contained in its Annual Report on Form 10-K for the year ended December 31, 2014, which is incorporated by reference into this prospectus supplement. The selected consolidated financial and operating data as of and for the three months ended March 31, 2015 and 2014 have been derived from Aon's unaudited condensed consolidated financial statements and related notes contained in Aon's Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2015, which is incorporated by reference into this prospectus supplement, except that the balance sheet data as of March 31, 2014 has been derived from Aon's unaudited condensed consolidated financial statements contained in its Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2014, which has not been incorporated by reference in this prospectus supplement. The results for the three months ended March 31, 2015 and 2014 are not necessarily indicative of the results that may be expected for the entire fiscal year. Aon's unaudited interim financial statements reflect all adjustments that management of Aon considers necessary for fair presentation of the financial position and results of operations for such periods in accordance with United States generally accepted accounting principles, which we refer to as GAAP.

        Historical results are not necessarily indicative of the results that may be expected for any future period. This selected consolidated financial and operating data should be read in conjunction with Aon's audited consolidated financial statements, the notes related thereto and "Management's Discussion and Analysis of Financial Condition and Results of Operations" contained in Aon's Annual Report on Form 10-K for the year ended December 31, 2014 and Aon's unaudited consolidated financial statements, the notes related thereto and "Management's Discussion and Analysis of Financial Condition and Results of Operations" contained in Aon's Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2015. See "Incorporation of Certain Documents by Reference" in this prospectus supplement.

 

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  Historical  
 
  Three Months
Ended March 31,
2015
  Three Months
Ended March 31,
2014
  Year Ended
December 31,
2014
  Year Ended
December 31,
2013
  Year Ended
December 31,
2012
 
 
  (millions, except shareholders, employees and per share data)
 

Income Statement Data

                               

Commissions, fees and other

  $ 2,842   $ 2,941   $ 12,019   $ 11,787   $ 11,476  

Fiduciary investment income

    5     6     26     28     38  

Total revenue

  $ 2,847   $ 2,947   $ 12,045   $ 11,815   $ 11,514  

Operating income

  $ 441   $ 469   $ 1,966   $ 1,671   $ 1,596  

Net Income(1)

  $ 341   $ 336   $ 1,431   $ 1,148   $ 1,020  

Less: Net income attributable to noncontrolling interests

    13     11     34     35     27  

Net income attributable to Aon shareholders

  $ 328   $ 325   $ 1,397   $ 1,113   $ 993  

Basic Net Income Per Share Attributable to Aon Shareholders

  $ 1.15   $ 1.07   $ 4.73   $ 3.57   $ 3.03  

Diluted Net Income Per Share Attributable to Aon Shareholders

  $ 1.14   $ 1.06   $ 4.66   $ 3.53   $ 2.99  

Balance Sheet Data

                               

Fiduciary assets(2)

  $ 10,894   $ 12,038   $ 11,638   $ 11,871   $ 12,214  

Intangible assets including goodwill

    10,958     11,530     11,380     11,575     11,918  

Total assets

    28,389     30,077     29,772     30,251     30,486  

Total debt

    5,676     4,746     5,582     4,389     4,165  

Total equity

    6,431     8,006     6,631     8,195     7,805  

Class A Ordinary Shares and Other Data

                               

Dividends paid per share

  $ 0.25   $ 0.18   $ 0.92   $ 0.68   $ 0.62  

Price range:

                               

High

    107.08     87.45     98.10     84.33     57.92  

Low

    89.35     76.49     76.49     54.65     45.04  

At period-end

                               

Market price

  $ 96.12   $ 84.28   $ 94.83   $ 83.89   $ 55.61  

Common shareholders

    252     266     255     281     240  

Shares outstanding

    281.7     296.5     280.0     300.7     310.9  

Number of employees

    68,982     66,029     68,633     65,547     64,725  

(1)
For the year ended December 31, 2012, amounts related to discontinued operations have been included in Other income to conform to amounts included in the Consolidated Financial Statements included in our Form 10-K for the years ended December 31, 2013 and 2014. These amounts in the year ended December 31, 2012, which were historically included in Income (loss) from discontinued operations, have been reclassified to conform with current presentation. The amounts reclassified were $1 million loss from Income (loss) from discontinued operations to Other income.

(2)
Represents insurance premiums receivables from clients as well as cash and investments held in a fiduciary capacity.

 

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RISK FACTORS

        You should carefully consider the risks described below, the risks set forth in the accompanying prospectus and the other information set forth in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein before making an investment decision. These risks include those set forth in the "Risk Factors" section of our Annual Report on Form 10-K for the year ended December 31, 2014 as filed with the SEC, and include risks that could have a material adverse effect on our and Aon Delaware's financial condition, results of operations or cash flows and which could, in turn, impact our and Aon Delaware's ability to perform our respective obligations under the Securities.

        Additional risks and uncertainties not presently known to us, or that we currently deem immaterial, may also impair our or Aon Delaware's business operations. The events discussed in the risk factors below, or the risk factors in the accompanying prospectus or the documents incorporated by reference herein or therein, may occur. If they do, our or Aon Delaware's business, results of operations or financial condition could be materially adversely affected. In such an instance, the trading prices of our or Aon Delaware's securities, including the Securities, could decline and you might lose all or part of your investment.

Risks Related to the Notes and the Guarantee

        The Notes are not secured by any of Aon plc's assets or the assets of its subsidiaries (including Aon Delaware), and the Guarantee is not secured by any of the assets of Aon Delaware or the assets of Aon Delaware's subsidiaries. As a result, the indebtedness represented by the Notes will effectively be subordinated to any existing and future secured indebtedness of Aon plc or its subsidiaries and the indebtedness represented by the Guarantee will effectively be subordinated to any existing and future secured indebtedness of Aon Delaware or its subsidiaries, in each case to the extent of the value of the assets securing any such indebtedness. As of March 31, 2015, neither Aon plc nor Aon Delaware had any secured indebtedness for borrowed money. As of March 31, 2015, Aon plc's subsidiaries (other than Aon Delaware) had approximately $1.6 billion of outstanding indebtedness and other liabilities, including trade payables, pensions and other post-employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. As of March 31, 2015, Aon Delaware's subsidiaries had approximately $1.4 billion of outstanding indebtedness and other liabilities, including trade payables, pensions and other post-employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. In the event of any distribution or payment of Aon plc's assets or those of Aon Delaware in any foreclosure, dissolution, winding up, liquidation or reorganization, or other bankruptcy proceeding, any secured creditors of Aon plc or of Aon Delaware, respectively, would have a superior claim to holders of the Notes to the extent of the value of their collateral. In the event of the dissolution, a winding up, liquidation or reorganization, or other bankruptcy proceeding of a subsidiary of Aon plc, creditors of that subsidiary would generally have the right to be paid in full before any distribution is made to Aon plc or you in respect of the Notes (except with respect to amounts payable by Aon Delaware under the Guarantee). In the event of a dissolution, winding up, liquidation or reorganization, or other bankruptcy proceeding of a subsidiary of Aon Delaware, creditors of that subsidiary would generally have the right to be paid in full before any distribution is made to Aon Delaware or to you in respect

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of the Guarantee. If any of the foregoing occur, we cannot assure you that there will be sufficient assets to pay amounts due on the Securities.

        Our liquidity is a function of our ability to successfully generate cash flows from operations and improvement therein, access to capital markets and borrowings under our credit agreements. We believe our liquidity (including operating and other cash flows that we expect to generate) will be sufficient to meet operating requirements as they occur; however, our ability to maintain sufficient liquidity going forward depends on our ability to generate cash from operations and access to the capital markets and borrowings, all of which are subject to general economic, financial, competitive, legislative, regulatory and other market factors that are beyond our control.

        We have a five-year $400 million unsecured revolving credit facility in the U.S. (the "2017 Facility"), which expires in 2017. Additionally, we have a five-year $900 million multi-currency credit facility available, which expires in February 2020 (the "2020 Facility"). Each of these credit facilities is intended to support our commercial paper obligations and our general working capital needs. At March 31, 2015, we had no borrowings outstanding under either of these facilities. At March 31, 2015, we were compliant with the financial covenants contained in our credit facilities. However, failure to comply with material provisions of our covenants in the credit facilities could result in a default under the credit agreements, rendering them unavailable to us and causing a material adverse effect on our or Aon Delaware's liquidity, results of operations and financial condition.

        The restrictive covenants in our financing agreements may impact how we operate our business and prevent us from engaging in certain potentially beneficial activities. For both the 2017 Facility and the 2020 Facility, the two most significant covenants require us to maintain a ratio of consolidated EBITDA (earnings before interest, taxes, depreciation and amortization), adjusted for (x) Hewitt-related transaction costs and up to $50 million in non-recurring cash charges, in the case of the 2017 Facility, and (y) up to $75 million in non-recurring cash charges, in the case of the 2020 Facility ("Adjusted EBITDA"), to consolidated interest expense and a ratio of consolidated debt to Adjusted EBITDA. For both facilities, the ratio of Adjusted EBITDA to consolidated interest expense must be at least 4.00 to 1.00. For the 2017 Facility, the ratio of consolidated debt to Adjusted EBITDA must not exceed the lower of (a) 3.25 to 1.00 or (b) the greater of (i) 3.00 to 1.00 and (ii) consolidated debt to Adjusted EBITDA then set forth in the 2020 Facility. For the 2020 Facility, the ratio of consolidated debt to Adjusted EBITDA must not exceed 3.25 to 1.00, subject to certain exceptions. The indenture governing the Securities does not include any similar covenants. Failure to comply with the covenants contained in the 2017 Facility, the 2020 Facility or any of our other existing indebtedness could result in an event of default under either such facility or our other existing indebtedness that, if not cured or waived, could have a material adverse effect on our or Aon Delaware's business, financial condition and results of operations. In the event of certain defaults under the 2017 Facility, the 2020 Facility or any of our other indebtedness, the lenders thereunder would not be required to lend any additional amounts to us and could elect to declare all borrowings outstanding, together with accrued and unpaid interest and fees, to be due and payable. If the indebtedness under the 2017 Facility, the 2020 Facility or any of our other indebtedness, including the Securities, were to be accelerated, there can be no

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assurance that our assets would be sufficient to repay such indebtedness in full. See "Description of the Securities."

        The indenture does not contain any financial covenants. The indenture will permit us and our subsidiaries (including Aon Delaware) to incur additional indebtedness, including secured indebtedness. Because the Securities will be unsecured, in the event of any dissolution, winding up, liquidation or reorganization or other bankruptcy proceeding regarding us or Aon Delaware, whether voluntary or involuntary, the holders of our or Aon Delaware's secured indebtedness will be entitled to receive payment to the extent of the value of the assets securing that indebtedness before we or Aon Delaware can make any payment with respect to the Securities. If any of the foregoing events occurs, we cannot assure you that we or Aon Delaware will have sufficient assets to pay amounts due on the Securities. As a result, you may receive a payment on the Securities that is less than that which you are entitled to receive or recover nothing if any liquidation, dissolution, reorganization, bankruptcy or other similar proceeding occurs.

        The indenture governing the Securities does not limit our, Aon Delaware's or our respective subsidiaries' ability to issue or repurchase securities, pay dividends, incur intercompany liabilities or engage in transactions with affiliates. Our and Aon Delaware's ability to use funds for numerous purposes may limit the funds available to pay our or Aon Delaware's obligations under the Securities.

        There can be no assurance regarding the future development of a market for the Securities or the ability of holders of the Securities to sell their Securities or the price at which such holders may be able to sell their Securities. If such a market were to develop, the Securities could trade at prices that may be higher or lower than the initial offering price depending on many factors, including, among other things, prevailing interest rates, our or Aon Delaware's operating results or financial condition and the market for similar securities. Underwriters, broker-dealers and agents that participate in the distribution of the Securities may make a market in the Securities as permitted by applicable laws and regulations but will have no obligation to do so, and any such market-making activities with respect to the Securities may be discontinued at any time without notice. Therefore, there can be no assurance as to the liquidity of any trading market for the Securities or that an active public market for the Securities will develop. See "Underwriting (Conflicts of Interest)." Aon plc intends to apply to list the Notes on the New York Stock Exchange or another "recognised stock exchange" for purposes of Section 1005 of the U.K. Income Tax Act 2007; however, there can be no assurance that the Notes will be so listed by the time the Notes are delivered to purchasers or at any time thereafter.

        The credit ratings of the Securities may not reflect the potential impact of all risks related to structure and other factors on any trading market for, or trading value of, the Securities. In addition, real or anticipated changes in our or Aon Delaware's credit ratings, which could result from any number of factors (including the modification by a credit rating agency of the criteria or methodology it applies to particular issuers, including us or Aon Delaware), will generally affect any trading market for, or trading value of, the Securities.

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USE OF PROCEEDS

        The net proceeds to us of this offering after deducting the underwriting discounts and estimated offering expenses payable by us, are expected to be approximately $            . We intend to use the net proceeds from this offering for general corporate purposes which, among other things, may include funding acquisition opportunities, repurchases of our Class A Ordinary Shares and repayment of all or part of our 3.50% senior notes due September 2015 (the "2015 Notes"). As of March 31, 2015, $600 million of our 2015 Notes was outstanding. Certain of the underwriters or their affiliates may hold the 2015 Notes and, accordingly, may receive proceeds from this offering. See "Underwriting (Conflicts of Interest)—Conflicts of Interest; Other Relationships."

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RATIO OF EARNINGS TO FIXED CHARGES

        Our ratios of earnings to fixed charges for each of the periods indicated are as follows:

 
  Three months ended
March 31,(1)
  Year ended December 31,(2)  
 
  2015   2014   2014   2013   2012   2011   2010  

Ratio of earnings to fixed charges

    7.0     6.9     6.7     6.7     6.0     5.6     5.6  

(1)
Refer to Exhibit 12.1 of our Quarterly Report on Form 10-Q for the quarter ended March 31, 2015 for the computation of these ratios.

(2)
Refer to Exhibit 12.1 of our Annual Report on Form 10-K for the year ended December 31, 2014 for the computation of these ratios.

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CAPITALIZATION

        The following table sets forth Aon plc's capitalization as of March 31, 2015, on an actual basis and on an as adjusted basis to give effect to this offering as if it had occurred on such date. You should read the data set forth in the table below in conjunction with "Summary—Selected Historical Financial Data" and "Use of Proceeds" appearing elsewhere in this prospectus supplement, as well as "Management's Discussion and Analysis of Financial Condition and Results of Operations," which is incorporated by reference into this prospectus supplement from our Quarterly Report on Form 10-Q for the quarter ended March 31, 2015.

 
  As of March 31, 2015  
 
  Actual   As Adjusted(1)  
 
  (Millions)
 

Cash and cash equivalents(1)

  $ 378   $    

Outstanding debt

             

        % Notes offered hereby

           

3.500% senior notes due June 2024

    600     600  

4.600% senior notes due June 2044

    550     550  

2.875% EUR 500 senior notes due May 2026

    545     545  

4.45% senior notes due May 2043

    250     250  

3.50% senior notes due September 2015

    600     600  

3.125% senior notes due May 2016

    500     500  

4.25% senior notes due December 2042

    166     166  

5.00% senior notes due September 2020

    600     600  

6.25% senior notes due September 2040

    300     300  

8.205% junior subordinated deferrable interest debentures due January 2027

    521     521  

4.76% CAD senior unsecured debentures due March 2018

    300     300  

4.00% senior notes due November 2023

    350     350  

Other

    394     394  

Total debt

    5,676        

Less short-term debt and current portion of long-term debt

    966     966  

Total long-term debt

    4,710        

Equity

             

Common stock—$0.01 nominal value

             

Authorized: 750 shares (issued: 281.7)

    3     3  

Additional paid-in capital

    5,177     5,177  

Retained earnings

    4,612     4,612  

Accumulated other comprehensive loss

    (3,427 )   (3,427 )

Total Aon Shareholders' Equity

    6,365     6,365  

Noncontrolling interests

    66     66  

Total Equity

    6,431     6,431  

Total capitalization

  $ 12,107   $ 12,107  

(1)
Cash and cash equivalents does not take into account expenses and discounts on issuance associated with this offering.

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DESCRIPTION OF THE SECURITIES

        The following description of the particular terms of the Securities offered by this prospectus supplement supplements, and to the extent inconsistent therewith, replaces the description of the general terms and provisions of the Securities set forth under the caption "Description of Debt Securities and Guarantees" in the accompanying prospectus. Terms used in this prospectus supplement that are otherwise not defined have the meanings given to them in the accompanying prospectus.

        Aon plc will issue $            aggregate principal amount of        % senior notes due 20        (the "Notes") pursuant to an amended and restated indenture to be dated on or around May     , 2015 among Aon plc, as issuer, Aon Delaware, as guarantor, and The Bank of New York Mellon Trust Company, N.A., as trustee (the "Trustee"). The following is a summary of the material provisions of the indenture governing the Securities. It does not include all of the provisions of the indenture. We urge you to read the indenture because it, not this description, defines your rights. The terms of the Securities include those stated in the indenture and those made part of the indenture by reference to the Trust Indenture Act of 1939, as amended (the "TIA"). A copy of the indenture may be obtained from Aon plc or the Trustee.

        Aon plc will issue the Notes in fully registered form in denominations of $2,000 and integral multiples of $1,000 in excess thereof. The Trustee will initially act as paying agent and registrar for the Notes. The Notes may be presented for registration of transfer and exchange at the offices of the registrar. Aon plc may change the paying agent and registrar without notice to holders of the Notes. It is expected that Aon plc will pay principal and interest (and premium, if any) on the Notes (and, as necessary, Aon Delaware will pay such amounts in relation to the Guarantee) at the Trustee's corporate office by wire transfer, if book-entry at DTC, or check mailed to the registered address of holders.

Principal, Maturity and Interest

        The Notes will mature on                        , 20        . $            in aggregate principal amount of Notes will be issued in this offering. After the issue date of the Notes, additional notes having the same terms and conditions as the Notes in all respects (other than the issue date, public offering price, and to the extent applicable, first date of interest accrual and first interest payment date of such notes) ("Additional Notes") may be issued from time to time; provided, however, that if the Additional Notes are not fungible with the Notes for U.S. federal income tax purposes, the Additional Notes will have a separate CUSIP number. The Notes and any Additional Notes that are actually issued will be treated as a single class for all purposes under the indenture, including, without limitation, as to waivers, amendments, redemptions and any applicable offers to purchase. Unless the context otherwise requires, for all purposes of the indenture and this "Description of the Securities," references to the Notes include any Additional Notes actually issued.

        Interest on the Notes will accrue at the rate of        % per annum and will be payable semi-annually in arrears in cash on each                        and                         , commencing on                         , 2015, to the persons who are registered holders at the close of business on the                        or                         , as the case may be, immediately preceding the applicable interest payment date. Interest on the Notes will accrue from the most recent date to which interest has been paid or, if no interest has been paid, from and including the date of issuance to but excluding the actual interest payment date.

        Interest will be computed on the basis of a 360-day year composed of twelve 30-day months.

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Optional Redemption

        Aon plc may at its option redeem all of the Notes at any time and some of the Notes from time to time, at a redemption price equal to the greater of:

plus, in either case, accrued and unpaid interest on the principal amount of the Notes being redeemed to but excluding the redemption date.

        On or after                        , 20        , Aon plc may redeem any or all of the Notes at a redemption price equal to 100% of the principal amount of the Notes being redeemed, plus accrued and unpaid interest on the principal amount of the Notes being redeemed to but excluding the redemption date.

        Notwithstanding the foregoing, installments of interest on Notes being redeemed that are due and payable on interest payment dates falling on or prior to a redemption date will be payable on the interest payment date to the registered holders as of the close of business on the relevant record date according to the Notes and the indenture.

        "Comparable Treasury Issue" means the United States Treasury security selected by the Quotation Agent as having a maturity comparable to the remaining term of the Notes to be redeemed that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of such Notes.

        "Comparable Treasury Price" means, with respect to any redemption date, (i) the average of three Reference Treasury Dealer Quotations for such redemption date, after excluding the highest and lowest such Reference Treasury Dealer Quotations, or (ii) if the Quotation Agent is given fewer than five such Reference Treasury Dealer Quotations, the average of all such quotations, or (iii) if only one Reference Treasury Dealer Quotation is received, such quotation.

        "Quotation Agent" means the Reference Treasury Dealer appointed by us.

        "Reference Treasury Dealer" means each of Morgan Stanley & Co. LLC, Barclays Capital Inc., HSBC Securities (USA) Inc. and J.P. Morgan Securities LLC (or their respective affiliates that are primary U.S. government securities dealers in New York City, each of which we refer to as a Primary Treasury Dealer) and their respective successors and any other nationally recognized investment banking firm that is a Primary Treasury Dealer appointed from time to time by us; provided, however, that if any of the foregoing shall cease to be a Primary Treasury Dealer, we will substitute therefor another Primary Treasury Dealer.

        "Reference Treasury Dealer Quotations" means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Quotation Agent, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Quotation Agent by such Reference Treasury Dealer at 5:00 p.m., New York City time, on the third business day preceding such redemption date.

        "Treasury Rate" means, with respect to any redemption date, the rate per annum equal to the semi-annual equivalent yield to maturity of the Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such redemption date.

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        Notice of any redemption described under "—Optional Redemption" will be mailed at least 30 days but not more than 90 days before the redemption date to each holder of the Notes to be redeemed. Unless we and Aon Delaware default in payment of the redemption price, on and after the redemption date, interest will cease to accrue on the Notes or portions thereof called for redemption. If less than all the Notes are to be redeemed, the Notes to be redeemed shall be selected by the Trustee using a method the Trustee deems to be appropriate and fair, and, if the Notes are in global form then in accordance with the procedures of the Depository Trust Company.

Guarantee

        Under the Guarantee, Aon Delaware will fully and unconditionally guarantee the due and punctual payment of the principal, interest, premium (if any) and all other amounts due under the Indenture and on the Notes when the Notes become due and payable, whether at maturity, pursuant to optional redemption, by acceleration or otherwise, in each case after any applicable grace periods or notice requirements, according to the terms of the Notes.

        The obligations of Aon Delaware under the Guarantee will be unconditional, regardless of the enforceability of the Notes, and will not be discharged until all obligations under the Notes and the indenture are satisfied. Holders of the Notes may proceed directly against Aon Delaware under the Guarantee if an event of default affecting the Notes occurs without first proceeding against Aon plc.

Ranking

        The Notes will be:

        The Guarantee will be:

        As noted above, the Notes will be effectively subordinated to all of Aon plc's existing and future secured indebtedness to the extent of the value of the assets securing any such indebtedness and structurally subordinated to all of Aon plc's subsidiaries' existing and future obligations (though you

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may have a direct claim as to Aon Delaware by virtue of its obligations with respect to the Guarantee). In addition, the Guarantee will be effectively subordinated to all of Aon Delaware's existing and future secured indebtedness to the extent of the value of the assets securing any such indebtedness and structurally subordinated to all of Aon Delaware's subsidiaries' existing and future obligations. See "Risk Factors—The Notes will be effectively subordinated to all of Aon plc's existing and future secured indebtedness (to the extent of the value of the assets securing any such indebtedness) and to the existing and future indebtedness of Aon plc's subsidiaries (though you may have a direct claim as to Aon Delaware by virtue of its obligations with respect to the Guarantee), and the Guarantee will be effectively subordinated to all of Aon Delaware's existing and future secured indebtedness (to the extent of the value of the assets securing any such indebtedness) and to the existing and future indebtedness of Aon Delaware's subsidiaries." As of March 31, 2015, Aon plc had no secured indebtedness for borrowed money and had approximately $11.1 billion of consolidated outstanding indebtedness and other liabilities, including trade payables, pension and other post employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. As of March 31, 2015 Aon plc's subsidiaries (excluding Aon Delaware) had approximately $1.6 billion of outstanding indebtedness and other liabilities, including trade payables, pension and other post employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. These liabilities (which exclude liabilities of Aon Delaware) constitute approximately 23.1% of Aon plc's total consolidated liabilities (excluding liabilities of Aon Delaware). As of March 31, 2015, Aon Delaware had no secured indebtedness for borrowed money and had approximately $4.3 billion of consolidated outstanding indebtedness and other liabilities, including trade payables, pension and other post employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities. As of March 31, 2015, Aon Delaware's subsidiaries had no secured indebtedness for borrowed money and had approximately $1.4 billion of outstanding indebtedness and other liabilities including trade payables, pension and other post employment liabilities, other current liabilities and non-current liabilities, but excluding intercompany liabilities and fiduciary liabilities, constituting approximately 24.2% of Aon Delaware's total consolidated liabilities.

Payment of Additional Amounts

        Payments made by Aon plc, Aon Delaware or a paying agent, as applicable, on the Notes or in respect of the Guarantee will be made free and clear of and without withholding or deduction for or on account of any present or future income, stamp or other tax, duty, levy, impost, assessment or other governmental charge of any nature whatsoever imposed or levied by or on behalf of the government of the United Kingdom or the United States (each, a "Home Country Jurisdiction"), of any territory of the Home Country Jurisdiction or by any authority or agency therein or thereof having the power to tax, which we refer to collectively as "Taxes," unless Aon plc, Aon Delaware or a paying agent is required to withhold or deduct Taxes by law.

        If Aon plc, Aon Delaware or a withholding agent is required to withhold or deduct any amount for or on account of Taxes from any payment made with respect to the Notes or the Guarantee, Aon plc or Aon Delaware, as applicable, will pay such additional amounts as may be necessary so that the net amount received by each beneficial owner (including additional amounts) after such withholding or deduction will not be less than the amount the beneficial owner would have received if the Taxes had not been withheld or deducted; provided that no additional amounts will be payable with respect to Taxes:

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nor shall additional amounts be paid with respect to any payment of the principal of or premium, if any, or interest, if any, on any Note or any payment in respect of the Guarantee to any such holder or beneficial owner who is a fiduciary or a partnership or a beneficial owner who is other than the sole beneficial owner of such payment to the extent a beneficiary or settlor with respect to such fiduciary or a member of such partnership or a beneficial owner would not have been entitled to such additional amounts had it been the holder of the Note.

        All references in this prospectus supplement and the accompanying prospectus, other than under "Description of Debt Securities and Guarantees—Defeasance" in the accompanying prospectus, to the payment of the principal of or premium, if any, or interest, if any, on or the net proceeds received on the sale or exchange of, any Notes or any payment made under the Guarantee shall be deemed to include additional amounts to the extent that, in that context, additional amounts are, were or would be payable.

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        Aon plc has agreed in the indenture that if it maintains a paying agent with respect to the Notes in any member state of the European Union, it will maintain a paying agent in at least one member state that will not be obliged to withhold or deduct taxes pursuant to European Council Directive 2003/48/EC or any other Directive amending, supplementing or replacing such Directive or any law implementing or complying with, or introduced in order to conform to such Directive or Directives, provided there is at least one member state that does not require a paying agent to withhold or deduct pursuant to such Directive.

        Aon plc's and Aon Delaware's obligations to pay additional amounts if and when due will survive the termination of the indenture and the payment of all other amounts in respect of the Notes.

        If, as a result of Aon plc's or Aon Delaware's consolidation, merger with or conversion into a successor person organized under the laws of a jurisdiction other than the United Kingdom or the United States (or, in each case, any political subdivision or taxing authority thereof) as described in the accompanying prospectus under "Description of Debt Securities and Guarantees—Consolidation and Merger," or the conveyance, transfer or lease by Aon plc or Aon Delaware of its assets substantially as an entirety to such successor person, and such an entity expressly assumes the obligations of Aon plc or Aon Delaware under the indenture and the Notes or the Guarantee, as applicable, such successor person will pay additional amounts on the same basis as described above, except that references to a "Home Country Jurisdiction" will be treated as references to the United Kingdom, the United States and the country in which such successor person is organized or resident (or deemed resident for tax purposes).

        Aon plc may redeem the Notes in whole, but not in part, at its option at any time prior to maturity, upon the giving of not less than 30 nor more than 90 days' notice of tax redemption to the holders, at a redemption price equal to the principal amount of the Notes plus accrued and unpaid interest, if any, to the redemption date, if:

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        No notice of any such redemption may be given earlier than 90 days prior to the earliest date on which Aon plc, Aon Delaware or such successor person, as applicable, would be obligated to pay any additional amounts.

        Aon plc, Aon Delaware or such successor person will also pay to each holder, or make available for payment to each such holder, on the redemption date, any additional amounts (as described under "—Payment of Additional Amounts") resulting from the payment of such redemption price by it. Prior to the delivery of any notice of redemption, Aon plc or such successor person will deliver to the Trustee an officer's certificate stating that it is entitled to effect or cause a redemption and setting forth a statement of facts showing that the conditions precedent of the right so to redeem or cause such redemption have occurred, and in the case of a redemption based on an opinion of independent counsel referred to in the second bullet above, such independent counsel's opinion. Delivery of any notice of redemption will be conclusive and binding on the holders of the Securities being redeemed.

        Any notice of redemption will be irrevocable once an officer's certificate has been delivered to the Trustee.

Governing Law

        The indenture and the Securities will be governed by, and construed in accordance with, the laws of the State of New York.

Concerning Our Relationship with the Trustee

        Aon Delaware and Aon plc have commercial deposits and custodial arrangements with The Bank of New York Mellon Trust Company, N.A., or "BNYM," and may have borrowed money from BNYM or its affiliates in the normal course of business. Aon Delaware and Aon plc may enter into similar or other banking relationships with BNYM or its affiliates in the future in the normal course of business. In addition, Aon Delaware and Aon plc have provided brokerage and other insurance services in the ordinary course of our business for BNYM or its affiliates. BNYM may also act as trustee with respect to other debt securities issued by Aon Delaware and Aon plc.

Offers to Purchase; Open Market Purchases

        Neither Aon Delaware nor Aon plc is required to make any sinking fund payments or any offers to purchase with respect to the Notes or the Guarantee. Aon Delaware or Aon plc may at any time and from time to time purchase Notes in the open market or otherwise.

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CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES

        The following is a general discussion of the material U.S. federal income tax consequences to beneficial owners of the Notes of the acquisition, ownership, and disposition of the Notes. This discussion is based upon the Code, the U.S. Treasury regulations promulgated thereunder, administrative pronouncements and judicial decisions, all as of the date hereof and all of which are subject to change, possibly on a retroactive basis.

        This discussion applies only to U.S. Holders (as defined below) that acquire the Notes in connection with their initial issuance at their initial offering price and hold the Notes as "capital assets" within the meaning of section 1221 of the Code. This discussion does not address all aspects of U.S. federal income taxation that might be important to particular investors in light of their individual circumstances or the U.S. federal income tax consequences applicable to special classes of taxpayers, such as banks and other financial institutions, insurance companies, real estate investment trusts, regulated investment companies, tax-exempt organizations, partnerships (or entities properly classified as partnerships for U.S. federal income tax purposes) or other pass-through entities, dealers in securities, traders in securities that elect to use a mark-to-market method of tax accounting, persons liable for U.S. federal alternative minimum tax, U.S. Holders whose functional currency is not the U.S. dollar, former citizens or residents of the United States, and persons holding the Notes as part of a conversion transaction or a straddle. The discussion does not address any foreign, state, local or non-income tax consequences of the acquisition, ownership or disposition of the Notes to beneficial owners of the Notes.

        As used in this prospectus supplement, the term "U.S. Holder" means a beneficial owner of a Note who or which is for U.S. federal income tax purposes:

        If a partnership or other entity properly classified as a partnership for U.S. federal income tax purposes is a beneficial owner of a Note, the tax treatment of a partner will generally depend upon the status of the partner and the activities of the entity. Such entities and partners in such entities should consult their own tax advisors about the U.S. federal income and other tax consequences of the acquisition, ownership and disposition of a Note.

        This discussion is for general purposes only. Holders should consult their own tax advisors regarding the application of the U.S. federal income tax laws to their particular situations and the consequences under federal estate or gift tax laws, as well as foreign, state or local laws and tax treaties, and the possible effects of changes in tax laws.

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U.S. Federal Income Taxation of U.S. Holders

        It is anticipated, and this discussion assumes, that the Notes will not be treated as issued with original issue discount for U.S. federal income tax purposes. Stated interest on Notes beneficially owned by a U.S. Holder generally will be taxable as ordinary interest income at the time payments are accrued or are received in accordance with the U.S. Holder's regular method of accounting for U.S. federal income tax purposes. Interest income on a Note will constitute foreign source income and will generally constitute "passive category income" for U.S. foreign tax credit purposes. Prospective purchasers should consult their tax advisors concerning the applicability of the foreign tax credit and source of income rules to income attributable to the Notes.

        Upon the sale, exchange, redemption or other taxable disposition of the Notes, a U.S. Holder generally will recognize gain or loss equal to the difference, if any, between (i) the amount realized upon the sale, exchange, redemption or other taxable disposition of the Notes, other than amounts attributable to accrued and unpaid interest (which will be taxed as ordinary interest income to the extent not previously included in income), and (ii) the U.S. Holder's adjusted tax basis in the Notes. The amount realized by a U.S. Holder is the sum of cash plus the fair market value of all other property received on the sale, exchange, redemption or other taxable disposition. A U.S. Holder's adjusted tax basis in the Notes generally will be its cost for the Notes.

        Gain or loss a U.S. Holder recognizes on the sale, exchange, redemption or other taxable disposition of a Note generally will be capital gain or loss. Such gain or loss generally will be long-term capital gain or loss if a U.S. Holder has held the Note for more than one year. For non-corporate U.S. Holders, long-term capital gains are taxed at a lower rate than ordinary income. The deductibility of capital losses is subject to limitations. A U.S. Holder should consult its own tax advisor regarding the deductibility of capital losses in its particular circumstances.

        In general, a U.S. Holder that is not an "exempt recipient" will be subject to U.S. federal backup withholding at the applicable rate (currently 28%) with respect to payments on the Notes and the proceeds of a sale, exchange, redemption or other taxable disposition of the Notes, unless the U.S. Holder provides its taxpayer identification number to the paying agent and certifies, under penalties of perjury, that it is not subject to backup withholding on an Internal Revenue Service Form W-9 (Request for Taxpayer Identification Number and Certification) and otherwise complies with the applicable requirements of the backup withholding rules. Backup withholding is not an additional tax. The amount of any backup withholding from a payment to a U.S. Holder may be allowed as a credit against such U.S. Holder's U.S. federal income tax liability and may entitle such U.S. Holder to a refund, provided the required information is furnished to the Internal Revenue Service in a timely manner. In addition, payments on the Notes made to, and the proceeds of a sale or other taxable disposition received by, a U.S. Holder that is not an exempt recipient generally will be subject to information reporting requirements.

        Certain U.S. Holders who are individuals, estates or trusts will be required to pay a 3.8% Medicare tax on all or part of their "net investment income," which includes, among other items, interest on, and capital gains from the sale or other taxable disposition of, a Note. Prospective investors should consult their tax advisors regarding the consequences, if any, with respect to this tax on their investment in the Notes.

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        Pursuant to legislation commonly referred to as FATCA, and regulations promulgated thereunder, foreign financial institutions must comply with information reporting rules with respect to their U.S. account holders and investors or bear a withholding tax on certain U.S. source payments made to them (including such payments made to them in their capacity as intermediaries). Generally, if a foreign financial institution or certain other foreign entity does not comply with these reporting requirements, "withholdable payments" to the noncomplying entity will be subject to a 30% withholding tax. For this purpose, withholdable payments are U.S.-source payments otherwise subject to nonresident withholding tax and, beginning in 2017, also include the entire gross proceeds from the sale of certain equity or debt instruments of U.S. issuers. Foreign entities located in jurisdictions that have entered into intergovernmental agreements with the United States in connection with FATCA may be subject to different rules.

        The Notes are not expected to pay U.S.-source amounts, so FATCA is not expected to apply to the Notes. However, in the event any withholding under FATCA is required or advisable with respect to any payments on the Notes, there will be no additional amounts payable to compensate for the withheld amount. Prospective purchasers should consult with their tax advisors regarding these new provisions.

        The U.S. federal income tax discussion set forth above is included for general information only and may not be applicable depending upon a Holder's particular situation. Prospective purchasers of the Notes should consult their own tax advisors with respect to the tax consequences to them of the acquisition, ownership and disposition of the Notes, including the tax consequences under state, local, estate, foreign and other tax laws and tax treaties and the possible effects of changes in U.S. or other tax laws.

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CERTAIN UNITED KINGDOM TAX CONSEQUENCES

        The following is a general summary of the U.K. withholding tax consequences and reporting requirements in relation to payments of interest on the Notes and payments in respect of the Guarantee and of the U.K. stamp duty and stamp duty reserve tax consequences of the issue or transfer of the Notes. This summary does not deal with other U.K. tax aspects of acquiring, holding or disposing of the Securities. This summary is based upon U.K. tax law and the published practice of HM Revenue and Customs ("HMRC") in effect on the date of this prospectus supplement and is subject to any change in law or practice which may take effect after that date (including with retrospective effect).

        Holders (or prospective holders) of Securities who are in any doubt as to their tax position should consult their professional advisors.

U.K. Withholding Tax

Payments of interest on the Notes

        If the Notes are and continue to be listed on a "recognised stock exchange" within the meaning of section 1005 of the U.K. Income Tax Act 2007, interest payments on the Notes may be made without withholding on account of U.K. tax. The Notes are intended to be listed on the New York Stock Exchange, which is currently a recognised stock exchange for these purposes, or on another recognised stock exchange.

        In all other circumstances, payments of interest on the Notes may be subject to withholding on account of U.K. income tax at the basic rate (currently 20 per cent), subject to such relief as may be available under the provisions of any applicable double tax treaty or any other relief or exemption that may apply.

Payments in respect of the Guarantee

        Depending on the correct analysis under U.K. tax law of payments in respect of the Guarantee, it is possible that such payments would be subject to withholding on account of U.K. income tax at the basic rate (currently 20 per cent), subject to such relief as may be available under the provisions of any applicable double tax treaty or any other exemption which may apply.

Additional Amounts

        In certain circumstances, additional amounts may be payable on the Notes and Guarantee payments in respect of U.K. withholding tax. See "Description of the Securities—Payment of Additional Amounts."

Provision of Information

        Holders of Securities should be aware HMRC have powers to obtain certain information and documents relating to the Securities, including in relation to interest and other payments derived from the Securities. This may include details of the beneficial owners of the Securities, the persons for whom the Securities are held and the persons to whom payments derived from the Securities are or may be paid. Information may be obtained from a range of persons, including registrars, the registered holders of the Securities, persons who make, receive or are entitled to receive payments derived from the Securities and persons by or through whom interest and payments treated as interest are paid or credited. Information obtained by HMRC may be provided to tax authorities in other jurisdictions.

Stamp duty and stamp duty reserve tax

        No U.K. stamp duty or stamp duty reserve tax should be payable on the issue or transfer of the Notes.

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EUROPEAN UNION SAVINGS TAX DIRECTIVE

        Under European Council Directive 2003/48/EC on the taxation of savings income in the form of interest payments (the "EU Savings Directive"), each member state of the European Union (the "EU") must provide to the tax authorities of another member state details of payments of interest (and other similar income) made by a person within its jurisdiction to, or for the benefit of, an individual (or certain other types of person) resident in that other member state. However, for a transitional period, Austria is instead required (unless it elects otherwise during that period) to operate a withholding system in relation to such payments at a current rate of 35%. The ending of such transitional period is dependent upon the conclusion of certain other agreements relating to information exchange with certain other countries. However, during that transitional period, withholding will not apply under the EU Savings Directive to a payment if the beneficial owner of that payment authorizes an exchange of information instead. A number of non-EU countries and territories have adopted similar measures (either provision of information or transitional withholding) in relation to payments made by a person within its jurisdiction to, or collected by such a person to, an individual resident in an EU member state.

        On March 24, 2014, the Council of the European Union adopted a directive amending the EU Savings Directive (the "Amending Directive"), amending and broadening the scope of the requirements described above. The Amending Directive requires member states to apply these new requirements from January 1, 2017, and if they were to take effect, the changes would expand the range of payments covered by the EU Savings Directive, in particular to include additional types of income payable on securities. The Amending Directive would also expand the circumstances in which payments that indirectly benefit an individual resident in a member state must be reported or be subject to withholding. This approach would apply to payments made to, or secured for, persons, entities or legal arrangements (including trusts) where certain conditions are satisfied, and may in some cases apply where the person, entity or arrangement is established or effectively managed outside of the EU.

        However, on March 18, 2015, the European Commission proposed the repeal of the EU Savings Directive from January 1, 2017 in the case of Austria and from January 1, 2016 in the case of all other member states (subject to on-going requirements to fulfill administrative obligations such as the reporting and exchange of information relating to, or accounting for withholding taxes on, payments made before those dates and to certain other transitional provisions in the case of Austria). This is to prevent overlap between the EU Savings Directive and a new automatic exchange of information regime to be implemented under European Council Directive 2011/16/EU on administrative cooperation in the field of taxation (as amended by European Council Directive 2014/107/EU). The proposal also provides that, if it proceeds, member states will not be required to apply the new requirements of the Amending Directive.

        If Aon plc maintains a paying agent in an EU member state with respect to the Notes, it is required to do so in a member state that will not be obliged to withhold or deduct tax pursuant to the EU Savings Directive or to such similar measures. Investors who are in any doubt as to their position should consult their professional advisers.

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BOOK-ENTRY, DELIVERY AND FORM

        We have obtained the information in this section concerning The Depository Trust Company ("DTC"), Clearstream Banking, société anonyme ("Clearstream, Luxembourg") and Euroclear Bank S.A./N.V., as operator of the Euroclear System ("Euroclear") and their book-entry systems and procedures from sources that we believe to be reliable. We take no responsibility for an accurate portrayal of this information. In addition, the description of the clearing systems in this section reflects our understanding of the rules and procedures of DTC, Clearstream, Luxembourg and Euroclear as they are currently in effect. Those systems could change their rules and procedures at any time.

        The Notes will initially be represented by one or more fully registered global notes. Each such global note will be deposited with, or on behalf of, DTC or any successor thereto and registered in the name of Cede & Co. (DTC's nominee). You may hold your interests in the global notes in the United States through DTC, or in Europe through Clearstream, Luxembourg or Euroclear, either as a participant in such systems or indirectly through organizations which are participants in such systems. Clearstream, Luxembourg and Euroclear will hold interests in the global notes on behalf of their respective participating organizations or customers through customers' securities accounts in Clearstream, Luxembourg's or Euroclear's names on the books of their respective depositaries, which in turn will hold those positions in customers' securities accounts in the depositaries' names on the books of DTC. Citibank, N.A. will act as depositary for Clearstream, Luxembourg and JPMorgan Chase Bank, N.A. will act as depositary for Euroclear.

        So long as DTC or its nominee is the registered owner of the global securities representing the Notes, DTC or such nominee will be considered the sole owner and holder of the Notes for all purposes of the Notes and the indenture. Except as provided below, owners of beneficial interests in the Notes will not be entitled to have the Notes registered in their names, will not receive or be entitled to receive physical delivery of the Notes in definitive form and will not be considered the owners or holders of the Notes under the indenture, including for purposes of receiving any reports delivered by us, Aon Delaware or the trustee pursuant to the indenture. Accordingly, each person owning a beneficial interest in a Note must rely on the procedures of DTC or its nominee and, if such person is not a participant, on the procedures of the participant through which such person owns its interest, in order to exercise any rights of a holder of Notes.

        Unless and until Aon plc issues the Notes in fully certificated, registered form under the limited circumstances described below under the heading "—Certificated Notes":

The Depository Trust Company

        DTC will act as securities depositary for the Notes. The Notes will be issued as fully registered Notes registered in the name of Cede & Co. DTC is:

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        DTC holds securities that its direct participants deposit with DTC. DTC facilitates the settlement among direct participants of securities transactions, such as transfers and pledges, in deposited securities through electronic computerized book-entry changes in direct participants' accounts, thereby eliminating the need for physical movement of securities certificates.

        Direct participants of DTC include securities brokers and dealers (including the underwriters), banks, trust companies, clearing corporations and certain other organizations. DTC is owned by a number of its direct participants. Indirect participants of DTC, such as securities brokers and dealers, banks and trust companies, can also access the DTC system if they maintain a custodial relationship with a participant, either directly or indirectly.

        Purchases of Notes under DTC's system must be made by or through direct participants, which will receive a credit for the Notes on DTC's records. The ownership interest of each beneficial owner is in turn to be recorded on the records of direct participants and indirect participants. Beneficial owners will not receive written confirmation from DTC of their purchase, but beneficial owners are expected to receive written confirmations providing details of the transaction, as well as periodic statements of their holdings, from the direct participants or indirect participants through which such beneficial owners entered into the transaction. Transfers of ownership interests in the Notes are to be accomplished by entries made on the books of participants acting on behalf of beneficial owners. Beneficial owners will not receive certificates representing their ownership interests in Notes, except as provided below in "—Certificated Notes."

        To facilitate subsequent transfers, all Notes deposited with DTC are registered in the name of DTC's nominee, Cede & Co. The deposit of Notes with DTC and their registration in the name of Cede & Co. effect no change in beneficial ownership. DTC has no knowledge of the actual beneficial owners of the Notes. DTC's records reflect only the identity of the direct participants to whose accounts such Notes are credited, which may or may not be the beneficial owners. The participants will remain responsible for keeping account of their holdings on behalf of their customers.

        Conveyance of notices and other communications by DTC to direct participants, by direct participants to indirect participants and by direct participants and indirect participants to beneficial owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time.

Book-Entry Format

        Under the book-entry format, the paying agent will pay interest or principal payments to Cede & Co., as nominee of DTC. DTC will forward the payment to the direct participants, who will then forward the payment to the indirect participants (including Clearstream, Luxembourg or Euroclear) or to you as the beneficial owner. You may experience some delay in receiving your payments under this system. Neither we, Aon Delaware, the trustee under the indenture nor any paying agent has any direct responsibility or liability for the payment of principal or interest on the Notes to owners of beneficial interests in the Notes.

        DTC is required to make book-entry transfers on behalf of its direct participants and is required to receive and transmit payments of principal, premium, if any, and interest on the Notes. Any direct participant or indirect participant with which you have an account is similarly required to make book-entry transfers and to receive and transmit payments with respect to the Notes on your behalf. We, Aon Delaware and the trustee under the indenture have no responsibility for any aspect of the actions of DTC, Clearstream, Luxembourg or Euroclear or any of their direct or indirect participants. In addition, we, Aon Delaware and the trustee under the indenture have no responsibility or liability for any aspect of the records kept by DTC, Clearstream, Luxembourg, Euroclear or any of their direct

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or indirect participants relating to or payments made on account of beneficial ownership interests in the Notes or for maintaining, supervising or reviewing any records relating to such beneficial ownership interests. We and Aon Delaware also do not supervise these systems in any way.

        The trustee will not recognize you as a holder under the indenture, and you can only exercise the rights of a holder indirectly through DTC and its direct participants. DTC has advised us that it will only take action regarding a Note if one or more of the direct participants to whom the Note is credited directs DTC to take such action and only in respect of the portion of the aggregate principal amount of the Notes as to which that participant or participants has or have given that direction. DTC can only act on behalf of its direct participants. Your ability to pledge Notes to non-direct participants, and to take other actions, may be limited because you will not possess a physical certificate that represents your Notes.

        Neither DTC nor Cede & Co. (nor such other DTC nominee) will consent or vote with respect to the Notes unless authorized by a direct participant in accordance with DTC's procedures. Under its usual procedures, DTC will mail an omnibus proxy to us as soon as possible after the record date. The omnibus proxy assigns Cede & Co.'s consenting or voting rights to those direct participants to whose accounts the Notes are credited on the record date (identified in a listing attached to the omnibus proxy).

        Clearstream, Luxembourg or Euroclear will credit payments to the cash accounts of Clearstream, Luxembourg customers or Euroclear participants in accordance with the relevant system's rules and procedures, to the extent received by its depositary. These payments will be subject to tax reporting in accordance with relevant United States tax laws and regulations. Clearstream, Luxembourg or the Euroclear Operator, as the case may be, will take any other action permitted to be taken by a holder under the indenture on behalf of a Clearstream, Luxembourg customer or Euroclear participant only in accordance with its relevant rules and procedures and subject to its depositary's ability to effect those actions on its behalf through DTC.

        DTC, Clearstream, Luxembourg and Euroclear have agreed to the foregoing procedures in order to facilitate transfers of the Notes among participants of DTC, Clearstream, Luxembourg and Euroclear. However, they are under no obligation to perform or continue to perform those procedures, and they may discontinue those procedures at any time.

Transfers Within and Among Book-Entry Systems

        Transfers between DTC's direct participants will occur in accordance with DTC rules. Transfers between Clearstream, Luxembourg customers and Euroclear participants will occur in accordance with its applicable rules and operating procedures.

        DTC will effect cross-market transfers between persons holding directly or indirectly through DTC, on the one hand, and directly or indirectly through Clearstream, Luxembourg customers or Euroclear participants, on the other hand, in accordance with DTC rules on behalf of the relevant European international clearing system by its depositary. However, cross-market transactions will require delivery of instructions to the relevant European international clearing system by the counterparty in that system in accordance with its rules and procedures and within its established deadlines (European time). The relevant European international clearing system will, if the transaction meets its settlement requirements, instruct its depositary to effect final settlement on its behalf by delivering or receiving securities in DTC, and making or receiving payment in accordance with normal procedures for same-day funds settlement applicable to DTC. Clearstream, Luxembourg customers and Euroclear participants may not deliver instructions directly to the depositaries.

        Because of time-zone differences, credits of securities received in Clearstream, Luxembourg or Euroclear resulting from a transaction with a DTC direct participant will be made during the

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subsequent securities settlement processing, dated the business day following the DTC settlement date. Those credits or any transactions in those securities settled during that processing will be reported to the relevant Clearstream, Luxembourg customer or Euroclear participant on that business day. Cash received in Clearstream, Luxembourg or Euroclear as a result of sales of securities by or through a Clearstream, Luxembourg customer or a Euroclear participant to a DTC direct participant will be received with value on the DTC settlement date but will be available in the relevant Clearstream, Luxembourg or Euroclear cash amount only as of the business day following settlement in DTC.

        Although DTC, Clearstream, Luxembourg and Euroclear has agreed to the foregoing procedures in order to facilitate transfers of debt securities among their respective participants, they are under no obligation to perform or continue to perform such procedures and such procedures may be discontinued at any time.

Certificated Notes

        Unless and until they are exchanged, in whole or in part, for Notes in definitive form in accordance with the terms of the Notes, the Notes may not be transferred except (1) as a whole by DTC to a nominee of DTC or (2) by a nominee of DTC to DTC or another nominee of DTC or (3) by DTC or any such nominee to a successor of DTC or a nominee of such successor.

        Aon plc will issue Notes to you or your nominees, in fully certificated registered form, rather than to DTC or its nominees, only if:

        If any of the three above events occurs, DTC is required to notify all direct participants that Notes in fully certificated registered form are available through DTC. DTC will then surrender the global notes representing the Notes along with instructions for re-registration. The trustee will re-issue the Notes in fully certificated registered form and will recognize the registered holders of the certificated debt securities as holders under the indenture.

        Unless and until Aon plc issues the Notes in fully certificated, registered form, (1) you will not be entitled to receive a certificate representing your interest in the Notes; (2) all references in this prospectus supplement or the accompanying prospectus to actions by holders will refer to actions taken by the depositary upon instructions from their direct participants; and (3) all references in this prospectus supplement or the accompanying prospectus to payments and notices to holders will refer to payments and notices to the depositary, as the registered holder of the Notes, for distribution to you in accordance with its policies and procedures.

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UNDERWRITING (CONFLICTS OF INTEREST)

        Under the terms and subject to the conditions contained in an underwriting agreement dated the date of this prospectus supplement, Aon plc and Aon Delaware have agreed to sell to the underwriters named below, for whom Morgan Stanley & Co. LLC, Barclays Capital Inc., HSBC Securities (USA) Inc. and J.P. Morgan Securities LLC are acting as representatives, the following respective principal amounts of the Securities.

Underwriter
  Principal Amount of
the Notes
 

Morgan Stanley & Co. LLC

  $    

Barclays Capital Inc. 

       

HSBC Securities (USA) Inc. 

       

J.P. Morgan Securities LLC

       

Total

  $    

        The underwriting agreement provides that the underwriters are obligated to purchase all of the Securities if any are purchased. The underwriting agreement also provides that if an underwriter defaults with respect to the Securities the purchase commitments of non-defaulting underwriters may be increased or the offering of the Securities may be terminated.

        The underwriters propose to offer the Securities initially at the applicable public offering price on the cover page of this prospectus supplement and to selling group members at that price less a selling concession of        % of the principal amount of the Securities. The underwriters may allow, and dealers may reallow, a concession not to exceed        % of the principal amount of the Securities on sales to other dealers. After the initial public offering the representatives may change the public offering price and concession and discount to broker/dealers.

        We estimate that our out of pocket expenses for this offering will be approximately $            .

        The Securities are a new issue of securities with no established trading market. We intend to apply to list the Notes on the New York Stock Exchange or another "recognised stock exchange" for purposes of Section 1005 of the U.K. Income Tax Act 2007; however, there can be no assurance that the Notes will be so listed by the time the Notes are delivered or at any time thereafter. In addition, one or more of the underwriters intend to make a secondary market for the Notes. However, they are not obligated to do so and may discontinue making a secondary market for the Notes at any time without notice. No assurance can be given as to how liquid the trading market for the Notes will be.

        We expect to deliver the Securities against payment for the Securities on or about the date specified in the last paragraph of the cover page of this prospectus supplement, which will be the fifth business day following the date of the pricing of the Securities. Under Rule 15c6-1 of the Exchange Act, trades in the secondary market generally are required to settle in three business days, unless the parties to any such trade expressly agree otherwise. Accordingly, purchasers who wish to trade Securities on the date of pricing or the next succeeding business day will be required, by virtue of the fact that the Securities initially will settle in T+5, to specify an alternate settlement arrangement to prevent a failed settlement.

        We have agreed that we will not offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, or file with the Securities and Exchange Commission a registration statement under the Securities Act of 1933 (the "Securities Act") relating to, any additional debt securities, or publicly disclose the intention to make any such offer, sale, pledge, disposition or filing, without the prior written consent of the representatives until the settlement date for the Securities.

        We have agreed to indemnify the several underwriters against liabilities under the Securities Act, or contribute to payments which the underwriters may be required to make in that respect.

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        In connection with the offering, the underwriters may engage in stabilizing transactions, over-allotment transactions, syndicate covering transactions and penalty bids in accordance with Regulation M under the Exchange Act.

        These stabilizing transactions, syndicate covering transactions and penalty bids may have the effect of raising or maintaining the market price of the Securities or preventing or retarding a decline in the market price of the Securities. As a result the price of the Securities may be higher than the price that might otherwise exist in the open market. These transactions, if commenced, may be discontinued at any time.

Conflicts of Interest; Other Relationships

        The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. From time to time in the ordinary course of their respective businesses, certain of the underwriters and their affiliates have engaged in and may in the future engage in commercial banking, derivatives and/or financial advisory, investment banking and other commercial transactions and services with us and our affiliates for which they have received or will receive customary fees and commissions.

        In addition, in the ordinary course of their business activities, the underwriters and their affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers. Such investments and securities activities may involve securities and/or instruments of ours or our affiliates. The underwriters and their affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or financial instruments and may hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

        Certain of the underwriters or their affiliates may hold the 2015 Notes and, accordingly, may receive proceeds from this offering. To the extent that any one underwriter, together with its affiliates, receives more than 5% of the net proceeds such underwriter would be deemed to have a "conflict of interest" with us in regard to this offering under FINRA Rule 5121. Accordingly, this offering will be made in compliance with the applicable provisions of FINRA Rule 5121.

European Economic Area

        This prospectus supplement and the accompanying prospectus are not prospectuses for the purposes of the Prospectus Directive as implemented in Member States of the European Economic

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Area. This prospectus supplement and the accompanying prospectus have been prepared on the basis that all offers of the Securities will be made pursuant to an exemption under the Prospectus Directive from the requirement to produce a prospectus in connection with offers of the Securities. Accordingly, any person making or intending to make any offer within the European Economic Area of the Securities which are the subject of the offering contemplated by this prospectus supplement and the accompanying prospectus should only do so in circumstances in which no obligation arises for us, Aon Delaware or any underwriter to produce a prospectus for such offers. None of we, Aon Delaware or the underwriters have authorized, nor do we or they authorize, the making of any offer of the Securities through any financial intermediary, other than offers made by underwriters or their affiliates which constitute the final placement of the Securities contemplated by this prospectus supplement and the accompanying prospectus.

        In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a "Relevant Member State") no offer of the Securities which are the subject of the offering contemplated by this prospectus supplement and the accompanying prospectus has been made or will be made to the public in that Relevant Member State other than:

provided that no such offer of Notes shall require us, Aon Delaware or any underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Directive.

        For the purposes of this provision, the expression an "offer of Securities to the public" in relation to any Securities which are the subject of the offering contemplated by this prospectus supplement and the accompanying prospectus in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and the Securities to be offered so as to enable an investor to decide to purchase or subscribe for such Securities, as the same may be varied in that Member State by any measure implementing the Prospectus Directive in that Member State, and the expression "Prospectus Directive" means Directive 2003/71/EC (as amended, including by Directive 2010/73/EU), and includes any relevant implementing measure in the Relevant Member State.

United Kingdom

        The communication of this prospectus supplement, the accompanying prospectus and any other document or materials relating to the issue of the Securities offered hereby is not being made, and such documents and/or materials have not been approved, by an authorized person for the purposes of section 21 of the United Kingdom's Financial Services and Markets Act 2000. Accordingly, such documents and/or materials are not being distributed to, and must not be passed on to, the general public in the United Kingdom. The communication of such documents and/or materials as a financial promotion is only being made to those persons in the United Kingdom falling within the definition of investment professionals (as defined in Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (the "Financial Promotion Order), or within Article 49(2)(a) to (d) of the Financial Promotion Order, or to any other persons to whom it may otherwise lawfully be made under the Financial Promotion Order (all such persons together being referred to as "relevant persons"). In the United Kingdom, the Securities offered hereby are only available to, and any investment or investment activity to which this offering circular relates will be engaged in only with, relevant persons. Any person in the United Kingdom that is not a relevant person should not act or rely on this prospectus supplement, the accompanying prospectus or any of the contents thereof.

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        All applicable provisions of the FSMA with respect to anything done in relation to the Securities in, from or otherwise involving the United Kingdom must be complied with.

Hong Kong

        This prospectus has not been approved by or registered with the Securities and Futures Commission of Hong Kong or the Registrar of Companies of Hong Kong. The Securities will not be offered or sold in Hong Kong other than (a) to "professional investors" as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) in other circumstances which do not result in the document being a "prospectus" as defined in the Companies Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. No advertisement, invitation or document relating to the Securities which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) has been issued or will be issued in Hong Kong or elsewhere other than with respect to Securities which are or are intended to be disposed of only to persons outside Hong Kong or only to "professional investors" as defined in the Securities and Futures Ordinance and any rules made under that Ordinance.

Japan

        The Securities have not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948, as amended) and, accordingly, will not be offered or sold, directly or indirectly, in Japan, or for the benefit of any Japanese Person or to others for re-offering or resale, directly or indirectly, in Japan or to any Japanese Person, except in compliance with all applicable laws, regulations and ministerial guidelines promulgated by relevant Japanese governmental or regulatory authorities in effect at the relevant time. For the purposes of this paragraph, "Japanese Person" shall mean any person resident in Japan, including any corporation or other entity organized under the laws of Japan.

Singapore

        This prospectus has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the Securities may not be circulated or distributed, nor may the Securities be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act (Chapter 289) (the "SFA"), (ii) to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA. Where the Securities are subscribed or purchased under Section 275 by a relevant person which is: (a) a corporation (which is not an accredited investor) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or (b) a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary is an accredited investor, then securities, debentures and units of securities and debentures of that corporation or the beneficiaries' rights and interest in that trust shall not be transferable for 6 months after that corporation or that trust has acquired the Securities under Section 275 except: (i) to an institutional investor under Section 274 of the SFA or to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA; (ii) where no consideration is given for the transfer; or (iii) by operation of law.

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INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

        The SEC allows us to "incorporate by reference" into this prospectus supplement the information we file with the SEC, which means that we can disclose important information to you by referring you to those documents. Any information incorporated this way is considered to be part of this prospectus supplement, and any information that we file later with the SEC will automatically update and supersede this information. SEC rules and regulations also allow us to "furnish" rather than "file" certain reports and information with the SEC. Any such reports or information which we have indicated as being "furnished" shall not be deemed to be incorporated by reference in or otherwise become a part of this prospectus, regardless of when furnished to the SEC. We incorporate by reference the following documents that we have filed with the SEC and any future filings that we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, which we refer to as the "Exchange Act" (other than information furnished rather than filed):

        You may request a copy of any filing referred to above (including any exhibits that are specifically incorporated by reference), at no cost, by contacting Aon at the following address or telephone number:

Aon plc
8 Devonshire Square
London, England EC2M 4PL
Attention: Company Secretary
Telephone: +44 20 7623 5500
  Aon Corporation
200 E. Randolph Street
Chicago, IL 60601
Attention: Company Secretary
Telephone: (312) 381-1000


LEGAL MATTERS

        The validity of the Securities will be passed upon for us as to U.S. law by Sidley Austin LLP, New York, New York. Davis Polk & Wardwell LLP, New York, New York, will pass upon certain matters for the underwriters.


EXPERTS

        The consolidated financial statements of Aon plc appearing in Aon plc's Annual Report (Form 10-K) for the year ended December 31, 2014, and the effectiveness of Aon plc's internal control over financial reporting as of December 31, 2014, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, included therein, and incorporated herein by reference. Such consolidated financial statements and Aon plc management's assessment of the effectiveness of internal control over financial reporting as of December 31, 2014 are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

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PROSPECTUS

LOGO

Aon plc

Debt Securities
Guarantees
Preference Shares
Class A Ordinary Shares
Convertible Securities
Share Purchase Contracts
Share Purchase Units



Aon Corporation

Debt Securities
Guarantees

        We may from time to time offer and sell any of our securities listed above, in each case, in one or more series. Our subsidiary, Aon Corporation, which we refer to as "Aon Delaware," also may from time to time offer and sell its debt securities in one or more series. Aon Delaware may guarantee all payments of principal, interest (if any), premium (if any), and other amounts due on any debt securities we issue. We may guarantee all payments of principal, interest (if any), premium (if any), and other amounts due on any debt securities Aon Delaware issues.

        This prospectus contains a general description of the securities that we or Aon Delaware may offer for sale. The specific terms of these securities will be contained in one or more supplements to this prospectus. Prospectus supplements may also add to, update or change information contained in this prospectus. You should read this prospectus and each applicable prospectus supplement, as well as the documents incorporated or deemed to be incorporated by reference in this prospectus and each applicable prospectus supplement, carefully before you invest.

        Investing in the securities described in this prospectus involves risk. You should carefully review the risks and uncertainties described under the heading "Risk Factors" contained in this prospectus and any risk factors set forth in each applicable prospectus supplement and in the documents incorporated or deemed to be incorporated by reference in this prospectus or any applicable prospectus supplement.

        Our executive offices are located at 8 Devonshire Square, London, England, EC2M 4PL, and our telephone number is +44 20 7623 5500. Aon Delaware's executive offices are located at 200 East Randolph Street, Chicago, Illinois 60601, and Aon Delaware's telephone number is (312) 381-1000.

        Our Class A Ordinary Shares are listed on the New York Stock Exchange under the symbol "AON."



        Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.



        We or Aon Delaware may sell securities directly or to or through underwriters, dealers or agents on a delayed or continuous basis. For additional information on the method of sale, you should refer to "Plan of Distribution" in this prospectus. The names of any underwriters, dealers or agents involved in the sale of any securities and the specific manner in which securities may be offered, including any applicable purchase price, fee, commission or discount arrangement between or among them, will be set forth in a prospectus supplement applicable to the sale of those securities.



The date of this prospectus is August 31, 2012.


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TABLE OF CONTENTS

About this Prospectus

    3  

Where You Can Find More Information

    4  

Information Concerning Forward-Looking Statements

    5  

Risk Factors

    6  

The Company

    7  

Use of Proceeds

    8  

Ratios

    9  

Description of Debt Securities and Guarantees

    10  

Description of Preference Shares

    26  

Description of Class A Ordinary Shares

    27  

Description of Share Purchase Contracts and Share Purchase Units

    27  

Plan of Distribution

    28  

Validity of Securities

    30  

Experts

    30  

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ABOUT THIS PROSPECTUS

        This prospectus is part of an automatic shelf registration statement that we filed with the Securities and Exchange Commission, which we refer to as the "SEC," as a "well-known seasoned issuer" as defined in Rule 405 under the Securities Act of 1933, as amended, which we refer to as the "Securities Act." Under the automatic shelf registration process, we may, over time, offer any combination of debt securities, guarantees, preference shares, Class A Ordinary Shares, convertible securities, share purchase contracts and share purchase units described in this prospectus in one or more offerings; and our subsidiary, Aon Delaware, may, over time, offer any combination of debt securities and guarantees described in this prospectus in one or more offerings. In this prospectus we refer to the debt securities, guarantees, preference shares, Class A Ordinary Shares, convertible securities, share purchase contracts and share purchase units offered by us, and the debt securities and guarantees offered by Aon Delaware, collectively as the "securities." This prospectus provides you with a general description of the securities we or Aon Delaware may offer. Each time we or Aon Delaware offer securities, we or Aon Delaware will provide you with one or more prospectus supplements that will contain specific information about the terms of those securities. A prospectus supplement may also add to, update or change the information contained in this prospectus. Please carefully read this prospectus and each applicable prospectus supplement, together with the documents incorporated or deemed to be incorporated by reference in this prospectus and the additional information described below under the heading "Where You Can Find More Information."

        As allowed by SEC rules, this prospectus does not contain all the information you can find in the registration statement of which this prospectus is a part or the exhibits to the registration statement. For further information, we refer you to the registration statement of which this prospectus is a part, including its exhibits and schedules. Statements contained in this prospectus about the provisions or contents of any contract, agreement or other document are not necessarily complete. For each of these contracts, agreements or documents filed as an exhibit to the registration statement, we refer you to the actual exhibit for a more complete description of the matters involved. You should rely only on the information contained or incorporated or deemed to be incorporated by reference in this prospectus and each applicable prospectus supplement. We have not authorized anyone else to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. You should not assume that the information contained or incorporated or deemed to be incorporated by reference in this prospectus or any applicable prospectus supplement is accurate as of any date other than the date on the cover of the applicable document. Our, or Aon Delaware's, business, financial condition and results of operations may have changed since that date. Neither this prospectus nor any prospectus supplement constitutes an offer to sell securities or a solicitation of an offer to buy securities by anyone in any jurisdiction in which that offer or solicitation is not authorized, or in which the person is not qualified to do so or to any person to whom it is unlawful to make that offer or solicitation.

        This prospectus is not intended to be and is not a prospectus for purposes of the E.U. Prospectus Directive and/or the U.K. Financial Services Authority's Prospectus Rules.

        In this prospectus, we use the terms "Aon plc," "Aon," the "Company," "we," "us" and "our" and similar terms to refer to Aon plc and its subsidiaries, unless the context otherwise requires and except as otherwise described below. We use the term "Aon Delaware" to refer to Aon Corporation, our wholly-owned subsidiary. On April 2, 2012, we completed the reorganization of the corporate structure of the group of companies controlled by Aon Delaware, Aon plc's predecessor as the ultimate holding company of the Aon group. In this prospectus, we refer to this transaction as the "Redomestication." In the Redomestication, each issued and outstanding share of Aon Delaware common stock held by stockholders of Aon Delaware was converted into the right to receive one Class A Ordinary Share, nominal value $0.01 per share, of Aon plc, subject to the receipt of cash for fractional shares. Any references in this prospectus to "Aon," the "Company," "we," "us" or "our" or any similar references relating to dates or periods before the Redomestication refer to Aon Delaware and its subsidiaries or, if the context so requires, Aon Delaware alone.

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WHERE YOU CAN FIND MORE INFORMATION

        We file annual, quarterly and current reports, proxy statements and other information with the SEC. You may read and copy any document we file with the SEC at the SEC's public reference room facility at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. You may obtain further information on the operation of the public reference room by calling the SEC at 1 800 SEC 0330. The SEC also maintains an Internet site at www.sec.gov that contains reports, proxy and information statements and other information regarding issuers, including us, that file documents with the SEC electronically through the SEC's electronic data gathering, analysis and retrieval system known as "EDGAR." In addition, you may inspect our SEC filings at the offices of the New York Stock Exchange, 20 Broad Street, New York, New York 10005.

        We make available free of charge most of our SEC filings through our Internet website (http://www.aon.com) as soon as reasonably practicable after we electronically file these materials with the SEC. You may access these SEC filings on our website. You may also find additional information about Aon plc, Aon Delaware and our other subsidiaries on our website. The information on, or accessible through, our web site is not a part of this prospectus. You may also request a copy of our SEC filings at no cost, by writing to or telephoning us at the following:

Aon plc
8 Devonshire Square
London, England EC2M 4PL
Attention: Company Secretary
Telephone: +44 20 7623 5500

        The SEC allows us to "incorporate by reference" into this prospectus the information we file with the SEC, which means that we can disclose important information to you by referring you to those documents. Any information incorporated this way is considered to be part of this prospectus, and any information that we file later with the SEC will automatically update and supersede this information. SEC rules and regulations also allow us to "furnish" rather than "file" certain reports and information with the SEC. Any such reports or information which we have indicated as being "furnished" shall not be deemed to be incorporated by reference in or otherwise become a part of this prospectus, regardless of when furnished to the SEC. We incorporate by reference the following documents that, following the Redomestication, we have or, prior to the Redomestication, Aon Delaware had filed with the SEC and any future filings that we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, which we refer to as the "Exchange Act," after the date of the initial filing of the registration statement of which this prospectus is a part until we complete our sale of the securities to the public (other than information in those filings that is furnished, under applicable SEC rules, rather than filed):

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INFORMATION CONCERNING FORWARD-LOOKING STATEMENTS

        This prospectus, the prospectus supplements, the documents incorporated or deemed to be incorporated by reference in this prospectus or any prospectus supplement and other written or oral statements made from time to time by us may contain certain statements related to future results, or state our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. They use words such as "anticipate," "believe," "estimate," "expect," "forecast," "project," "intend," "plan" and "potential," and other similar terms, and future or conditional tense verbs like "could," "may," "might," "should," "will" and "would." You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: market and industry conditions, including competitive and pricing trends; changes in our business strategies and methods of generating revenue; the development and performance of our services and products; changes in the composition or level of our revenues; our cost structure and the outcome of cost-saving or restructuring initiatives; the outcome of contingencies; dividend policy; the expected impact of acquisitions and dispositions; pension obligations; cash flow and liquidity; future actions by regulators; and the impact of changes in accounting rules. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include:

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Any or all of our forward-looking statements may turn out to be inaccurate, and there are no guarantees about our performance. The factors identified above are not exhaustive. Aon and its subsidiaries operate in a dynamic business environment in which new risks may emerge frequently. Accordingly, you should not place undue reliance on forward-looking statements, which speak only as of the dates on which they are made. We are under no obligation (and expressly disclaim any obligation) to update or alter any forward-looking statement that we may make from time to time, whether as a result of new information, future events or otherwise. Further information about factors that could materially affect Aon, including our results of operations and financial condition, is contained in the "Risk Factors" section in Part I, Item 1A of our Annual Report on Form 10-K for the year ended December 31, 2011 and in our most recently filed Quarterly Reports on Form 10-Q, and any amendments thereto.


RISK FACTORS

        An investment in our securities involves significant risks. Before making an investment decision, you should carefully consider the risks and other information we include or incorporate by reference in this prospectus or an applicable prospectus supplement. In particular, you should consider the risk factors set forth in any applicable prospectus supplement and in our most recent Annual Report on Form 10-K filed with the SEC, as those risk factors may be amended or supplemented by subsequent Quarterly Reports on Form 10-Q or otherwise. The risks and uncertainties we have described are not the only ones facing us. Additional risks and uncertainties not known to us or that we deem immaterial may also affect our business operations.

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THE COMPANY

        We are a preeminent professional service firm, focused on the topics of risk and people. We are the leading global provider of risk management services, insurance and reinsurance brokerage, and human resource consulting and outsourcing, delivering distinctive client value via innovative and effective risk management and workforce productivity solutions. We serve clients through two operating segments, Risk Solutions and HR Solutions. Risk Solutions acts as an advisor and insurance and reinsurance broker, helping clients manage their risks via consultation, as well as negotiation and placement of insurance risk with insurance carriers through our global distribution network. HR Solutions partners with organizations to solve their most complex benefits, talent and related financial challenges, and improve business performance by designing, implementing, communicating and administering a wide range of human capital, retirement, investment management, health care, compensation and talent management strategies. As of December 31, 2011, we had approximately 62,000 employees and conducted our operations through various subsidiaries in more than 120 countries and sovereignties.

        Aon Delaware is a wholly-owned Delaware subsidiary of Aon Holdings LLC, which is a wholly-owned direct subsidiary of Aon plc. Prior to the Redomestication, Aon Delaware was the ultimate holding company for the Aon group. See "About this Prospectus" and "Where You Can Find More Information" in this prospectus.

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USE OF PROCEEDS

        Unless we state otherwise in an applicable prospectus supplement, we expect to use the net proceeds from the sale of the securities described in this prospectus and an applicable prospectus supplement for general corporate purposes, including securities repurchase programs, capital expenditures, working capital, repayment or reduction of long term and short term debt and the financing of acquisitions. We may invest funds that we do not immediately require in short term marketable securities.

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RATIOS

        Our ratios of earnings to fixed charges and of earnings to combined fixed charges and preference share dividends for each of the periods indicated are as follows:

 
  Six
Months
Ended
June 30,
  Year Ended December 31,  
 
  2012   2011   2011   2010   2009   2008   2007  

Ratio of earnings to fixed charges

    6.0     5.6     5.6     5.6     6.5     6.1     5.7  

Ratio of earnings to combined fixed charges and preference share dividends

   
6.0
   
5.6
   
5.6
   
5.6
   
6.5
   
6.1
   
5.7
 

        For these ratios, earnings consist of income from continuing operations before provision for income taxes and noncontrolling interest, less the earnings from unconsolidated entities under the equity method of accounting, and fixed charges. Fixed charges include interest expense and that portion of rental expense we deem to represent interest. Combined fixed charges and preferred stock dividends include preference share dividend requirements, interest expense and that portion of rental expense we deem to represent interest. Preference share dividends consist of the pre tax earnings required to pay dividends on all preference shares.

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DESCRIPTION OF DEBT SECURITIES AND GUARANTEES

        In this description, references to "Aon," the "company," "we," "us" or "our" refer only to Aon plc and not to any of our subsidiaries or affiliates, including Aon Delaware. Also, in this section, references to "holders" mean those who own debt securities and the related guarantees registered in their own names, on the books that the appropriate registrar for Aon plc or Aon Delaware, as the case may be, maintains for this purpose, and not those who own beneficial interests in debt securities and the related guarantees registered in "street name" or in debt securities and the related guarantees issued in book-entry form and held through one or more depositaries.

        This prospectus describes certain general terms and provisions of the debt securities that we may offer (the "Aon plc debt securities") and that Aon Delaware may offer (the "Aon Delaware debt securities") pursuant to this prospectus. When we or Aon Delaware offer to sell a particular series of debt securities, we or Aon Delaware will describe the specific terms of the series in one or more prospectus supplements. We or Aon Delaware will also indicate in an applicable prospectus supplement the extent to which the general terms and provisions described in this prospectus apply to a particular series of debt securities.

        Aon plc may issue Aon plc debt securities under either (1) a senior indenture (the "Aon plc senior indenture") among Aon plc, as issuer; Aon Delaware, as guarantor (the "Aon plc senior debt guarantor") in respect of certain series of Aon plc senior debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the "Aon plc senior debt trustee"), or (2) a subordinated indenture (the "Aon plc subordinated indenture") among Aon plc, as issuer; Aon Delaware, as guarantor (the "Aon plc subordinated debt guarantor" and, together with the Aon plc senior debt guarantor, the "Aon plc guarantor") in respect of certain series of Aon plc subordinated debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the "Aon plc subordinated debt trustee" and, together with the Aon plc senior debt trustee, the "Aon plc trustee"). Any Aon plc debt securities that we issue under the Aon plc senior indenture will constitute unsubordinated debt of Aon plc ("Aon plc senior debt securities") and will rank senior to any Aon plc debt securities that Aon plc issues under the Aon plc subordinated indenture ("Aon plc subordinated debt securities"). Any guarantee that Aon Delaware, as the Aon plc senior debt guarantor, issues under the Aon plc senior indenture will constitute an unsubordinated obligation of Aon Delaware (each, an "Aon plc senior debt guarantee") and will rank senior to any guarantee that Aon Delaware, as the Aon plc subordinated debt guarantor, issues under the Aon plc subordinated indenture (each, an "Aon plc subordinated debt guarantee" and, together with the Aon plc senior debt guarantees, the "Aon plc debt guarantees").

        Aon Delaware may issue Aon Delaware debt securities under either (1) a senior indenture (the "Aon Delaware senior indenture") among Aon Delaware, as issuer; Aon plc, as guarantor (the "Aon Delaware senior debt guarantor") in respect of certain series of Aon Delaware plc senior debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the "Aon Delaware senior debt trustee"), or (2) a subordinated indenture (the "Aon Delaware subordinated indenture") among Aon Delaware, as issuer; Aon plc, as guarantor (the "Aon Delaware subordinated debt guarantor" and, together with the Aon Delaware senior debt guarantor, the "Aon Delaware guarantor") in respect of certain series of Aon plc Delaware subordinated debt securities (as defined below); and The Bank of New York Mellon Trust Company, N.A., as trustee (the "Aon Delaware subordinated debt trustee" and, together with the Aon Delaware senior debt trustee, the "Aon Delaware trustee"). Any Aon Delaware debt securities that Aon Delaware issues under the Aon Delaware senior indenture will constitute unsubordinated debt of Aon Delaware ("Aon Delaware senior debt securities") and will rank senior to any Aon Delaware debt securities that Aon Delaware issues under the Aon Delaware subordinated indenture ("Aon Delaware subordinated debt securities"). Any guarantee that Aon plc, as the Aon Delaware senior debt guarantor, issues under the Aon Delaware senior indenture will constitute an unsubordinated obligation of Aon plc (each, an "Aon Delaware

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senior debt guarantee") and will rank senior to any guarantee that Aon plc, as the Aon Delaware subordinated debt guarantor, issues under the Aon Delaware subordinated indenture (each, an "Aon Delaware subordinated debt guarantee" and, together with the Aon Delaware senior debt guarantees, the "Aon Delaware debt guarantees").

        In this description, the Aon plc debt securities and the Aon Delaware debt securities are sometimes referred to together as the "debt securities," the Aon plc senior debt securities and the Aon Delaware senior debt securities are sometimes referred to together as the "senior debt securities," the Aon plc subordinated debt securities and the Aon Delaware subordinated debt securities are sometimes referred together as the "subordinated debt securities," the Aon plc senior indenture and the Aon Delaware senior indenture are sometimes referred to together as the "senior indentures," the Aon plc subordinated indenture and the Aon Delaware subordinated indenture are sometimes referred to together as the "subordinated indentures," the senior indentures and the subordinated indentures are sometimes referred to together as the "indentures," the Aon plc debt guarantees and the Aon Delaware debt guarantees are sometimes referred to together as the "guarantees," each of Aon plc and Aon Delaware, in each case in its capacity as issuer of debt securities, is sometimes referred to as the "issuer," each of the Aon plc debt guarantor and the Aon Delaware debt guarantor is sometimes referred to as the "guarantor," each of the Aon plc senior trustee and the Aon Delaware senior trustee is sometimes referred to as the "senior trustee," each of the Aon plc subordinated trustee and the Aon Delaware subordinated trustee is sometimes referred to as the "subordinated trustee," and each of the senior trustee and the subordinated trustee is sometimes referred to as the "trustee."

        Each series of debt securities will be issued under the terms of an amendment or supplement to the applicable indenture that takes the form of a supplemental indenture or an officers' certificate delivered under the authority of resolutions adopted by the board of directors of the issuer and the terms of that indenture. The terms of any debt securities and, if applicable, the guarantees will include those stated in the applicable indenture and those made part of that indenture by reference to the Trust Indenture Act of 1939, which we refer to as the "Trust Indenture Act." The debt securities will be subject to all those terms, and we refer prospective purchasers and holders of debt securities and guarantees to the applicable indenture and the Trust Indenture Act for a statement of those terms.

        The following summaries of various provisions of the debt securities, the indentures and the guarantees are not complete. They do not describe certain exceptions and qualifications contained in the debt securities, the indentures and the guarantees, and are qualified in their entirety by reference to the provisions of the debt securities, the indentures and the guarantees. Unless we otherwise indicate, capitalized terms have the meanings assigned to them in the applicable indenture.

        An applicable prospectus supplement will specify the issuer, the guarantor, if any, whether the debt securities offered thereby will be senior or subordinated debt and whether the debt securities are to be guaranteed. The debt securities may be issued as part of units consisting of debt securities and other securities that Aon plc or Aon Delaware may offer under this prospectus. If debt securities are issued as part of units of debt securities and other securities that Aon plc or Aon Delaware may issue under this prospectus, an applicable prospectus supplement will describe any applicable material federal income tax consequences to holders.

General

        The debt securities will be unsecured obligations of the applicable issuer. None of the indentures limit the amount of debt securities that the issuer may issue. Each indenture provides that the issuer may issue debt securities from time to time in one or more series.

        The Aon plc senior debt securities and any Aon Delaware senior debt guarantee will be unsecured and unsubordinated obligations of Aon plc and will rank equally in right of payment with Aon plc's other unsecured and unsubordinated obligations. The Aon plc subordinated debt securities and any

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Aon Delaware subordinated debt guarantee will be subordinated obligations and will rank junior in right of payment, as more fully described in the applicable subordinated indenture, to Aon plc's senior indebtedness. Because Aon plc is a holding company, the holders of Aon plc debt securities and Aon Delaware debt guarantees may not receive assets of our subsidiaries in a liquidation or recapitalization until the claims of our subsidiaries' creditors and any insurance policyholders (in the case of our insurance subsidiaries) are paid, except to the extent that Aon plc may have recognized claims against such subsidiaries. In addition, certain regulatory laws limit some of Aon plc's subsidiaries from making payments to Aon plc of dividends and on loans and other transfers of funds.

        The Aon Delaware senior debt securities and any Aon plc senior debt guarantee will be unsecured and unsubordinated obligations of Aon Delaware and will rank equally in right of payment with Aon Delaware's other unsecured and unsubordinated obligations. The Aon Delaware subordinated debt securities and any Aon plc subordinated debt guarantee will be subordinated obligations and will rank junior in right of payment, as more fully described in the applicable subordinated indenture, to Aon Delaware's senior indebtedness. Because Aon Delaware is a holding company, the holders of Aon Delaware debt securities and Aon plc debt guarantees may not receive assets of Aon Delaware's subsidiaries in a liquidation or recapitalization until the claims of Aon Delaware's subsidiaries' creditors and any insurance policyholders (in the case of Aon Delaware's insurance subsidiaries) are paid, except to the extent that Aon Delaware may have recognized claims against such subsidiaries. In addition, certain regulatory laws limit Aon Delaware's subsidiaries from making payments to Aon Delaware of dividends and on loans and other transfers of funds.

        An applicable prospectus supplement will describe the specific terms relating to the series of debt securities being offered. These terms will include some or all of the following:

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        If an issuer denominates the purchase price of a series of debt securities in a non-United States dollar currency or currencies or a non-United States dollar currency unit or units, or if the principal of, any premium and interest on any series of debt securities is payable in a non-United States dollar currency or currencies or a non-United States dollar currency unit or units, an applicable prospectus supplement will describe any special United States federal income tax considerations.

        The issuer will pay principal and any interest, premium and additional amounts in the manner, at the places and subject to the restrictions set forth in the applicable debt securities, the applicable indenture and any applicable prospectus supplement. The issuer will not impose a service charge for any transfer or exchange of debt securities, but it may require payment of a sum sufficient to cover any tax or other governmental charge that may be imposed. (Section 2.05 of the indentures)

        Unless otherwise indicated in an applicable prospectus supplement, each issuer will issue debt securities in fully registered form, without coupons, in denominations of $1,000 or multiples of $1,000. (Sections 2.01 and 2.04 of the indentures)

        The issuer may offer to sell at a substantial discount below their stated principal amount, debt securities bearing no interest or interest at a rate that, at the time of issuance, is below the prevailing market rate. An applicable prospectus supplement will describe any special United States federal income tax considerations applicable to any of those discounted debt securities.

        The issuer may offer to sell debt securities in which the principal or interest will be determined by reference to one or more currency exchange rates, commodity prices, equity indices or other factors. The principal amount or payment of interest applicable to those debt securities may be greater than or less than the amount of principal or interest otherwise payable, depending upon the value of the applicable currency, commodity, equity index or other factor on the date on which that principal or interest is due. An applicable prospectus supplement will describe the methods used to determine the amount of principal or interest payable on any date, the currencies, commodities, equity indices or other factors to which the amount payable on that date is linked and certain additional tax considerations applicable to those debt securities.

        The indentures do not restrict our or Aon Delaware's ability to incur unsecured indebtedness or, subject to the restrictions described in "—Consolidation and Merger," to engage in reorganizations, restructurings, mergers, consolidations or similar transactions that have the effect of increasing our or Aon Delaware's indebtedness. Accordingly, unless an applicable prospectus supplement states otherwise, neither the debt securities nor any guarantees will contain any provisions that afford holders protection against the issuer or, if applicable, the guarantor incurring unsecured indebtedness or engaging in certain reorganizations or transactions. As a result, we or Aon Delaware could become highly leveraged.

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Events of Default

        With respect to any series of debt securities, "event of default" means any of the following:

        If there is a continuing event of default with respect to any outstanding series of debt securities, the applicable trustee or the holders of at least 25% of the outstanding principal amount of the debt securities of that series may require the issuer or, if applicable, the guarantor to pay immediately the principal (or, if the debt securities of that series are discount securities, that portion of the principal amount as may be specified in the terms of that series) of and accrued and unpaid interest, if any, on all debt securities of that series. However, at any time after that trustee or the holders, as the case may be, declare that acceleration with respect to debt securities of any series, but before the applicable person has obtained a judgment or decree for payment of the money, the holders of a majority in principal amount of the outstanding debt securities of that series may, under certain conditions, cancel such acceleration if (i) all events of default (other than the non-payment of accelerated principal) with respect to debt securities of that series have been cured or (ii) all such events of default have been waived, each as provided in the applicable indenture. (Section 6.01 of the indentures) For information as to waiver of defaults, see "—Modification and Waiver." The particular provisions relating to acceleration of the maturity of a portion of the principal amount of such debt securities that are discount securities triggered by an event of default shall be described in an applicable prospectus supplement.

        Each indenture provides that, subject to the duties of the applicable trustee to act with the required standard of care if there is a continuing event of default, the applicable trustee need not exercise any of its rights or powers under the indenture at the request or direction of any of the holders of debt securities, unless those holders have offered to the applicable trustee security or indemnity reasonably satisfactory to it. (Section 6.04 of the indentures) Subject to those provisions for

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security or indemnification of the applicable trustee and certain other conditions, the holders of the majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the applicable trustee or exercising any trust or power that trustee holds, in each case, with respect to the debt securities of that series. (Section 6.06 of the indentures)

        No holder of any debt security of any series will have any right to institute any proceeding with respect to any indenture or for any remedy under the applicable indenture unless:

        However, the holder of any debt security will have an absolute and unconditional right to receive payment of the principal of, and any premium or interest on, that debt security on or after the date or dates they are to be paid as expressed in or pursuant to that debt security and to institute suit for the enforcement of any such payment. (Section 6.04 of the indentures)

        Each indenture provides that the applicable trustee shall provide notice to the holders of debt securities of any series within 90 days of the occurrence of any default with respect to debt securities of that series known to such trustee, except that the trustee need not provide holders of debt securities of any series notice of any default (other than the non-payment of principal or any premium, interest or additional amounts) if such default has been cured and it considers it in the interest of the holders of debt securities of that series not to provide that notice. (Section 6.07 of the indentures)

Consolidation and Merger

        Each indenture provides that each of the issuer and the guarantor may consolidate with or merge or convert into, or convey, transfer or lease its properties or assets substantially as an entirety to, another person without the consent of any debt security holders if, along with certain other conditions set forth in the indentures:

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        It is possible that a merger, transfer, lease or other transaction could be treated for United States federal income tax purposes as a taxable exchange by the holders of debt securities or guarantees for new securities, which could result in holders of debt securities or guarantees recognizing taxable gain or loss for US federal income tax purposes. A merger, transfer, lease or other transaction could also have adverse tax consequences to holders of debt securities or guarantees under other tax laws to which the holders are subject.

Payment of Additional Amounts by Aon plc

        Payments made by Aon plc as issuer under or with respect to the Aon plc debt securities and as Aon Delaware debt guarantor under or with respect to the Aon Delaware debt guarantees will be made free and clear of and without withholding or deduction for or on account of any present or future income, stamp or other tax, duty, levy, impost, assessment or other governmental charge of any nature whatsoever imposed or levied by or on behalf of the government of the United Kingdom, of any territory of the United Kingdom or by any authority or agency therein or thereof having the power to tax, which we refer to collectively as "Taxes," unless Aon plc is required to withhold or deduct Taxes by law.

        If Aon plc is required to withhold or deduct any amount for or on account of Taxes from any payment made with respect to the Aon plc debt securities or Aon Delaware debt guarantees, Aon plc will pay such additional amounts as may be necessary so that the net amount received by each holder (including additional amounts) after such withholding or deduction will not be less than the amount the holder would have received if the Taxes had not been withheld or deducted; provided that no additional amounts will be payable with respect to Taxes:

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nor shall additional amounts be paid with respect to any payment of the principal of or interest on any Aon plc debt security or Aon Delaware debt guarantees to any such holder who is a fiduciary or a partnership or a beneficial owner who is other than the sole beneficial owner of such payment to the extent a beneficiary or settlor with respect to such fiduciary or a member of such partnership or a beneficial owner would not have been entitled to such additional amounts had it been the holder of the debt security.

        All references in this prospectus, other than under "—Defeasance" below, to the principal of or premium, if any, or interest, if any, on or the net proceeds received on the sale or exchange of, any debt securities shall be deemed to include additional amounts to the extent that, in that context, additional amounts are, were or would be payable.

        Aon plc has agreed in the Aon plc senior indenture and the Aon plc subordinated indenture that at least one paying agent for each series of Aon plc debt securities will be located outside the United Kingdom. Aon plc has also agreed that if it maintains a paying agent with respect to a particular series of Aon plc debt securities in any member state of the European Union, it will maintain a paying agent in at least one member state (other than the United Kingdom) that will not be obliged to withhold or deduct taxes pursuant to any law implementing European Council Directive 2003/48/EC or any other Directive implementing the conclusions of the ECOFIN Council meeting of November 26-27, 2000 on the taxation of savings income, provided there is at least one member state that does not require a paying agent to withhold or deduct pursuant to such Directive.

        Aon plc's obligation to pay additional amounts if and when due will survive the termination of the indentures and the payment of all other amounts in respect of the debt securities.

        If, as a result of Aon plc's consolidation, merger with or conversion into a successor person organized under the laws of a jurisdiction other than the United Kingdom (or, in each case, any political subdivision or taxing authority thereof) as described above under "—Consolidation and Merger", or the conveyance, transfer or lease by Aon plc of its assets substantially as an entirety to such successor person, and such an entity expressly assumes the obligations of Aon plc under the indentures and any outstanding Aon plc debt securities and Aon Delaware debt guarantees, such successor person will pay additional amounts on the same basis as described above, except that references to the "United Kingdom" will be treated as references to both the United Kingdom and the country in which such successor person is organized or resident (or deemed resident for tax purposes).

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Optional Tax Redemption

        In the case of Aon plc debt securities and Aon Delaware debt securities benefiting from an Aon Delaware debt guarantee, the issuer may redeem any series of those debt securities in whole but not in part at any time, on giving not less than 30 nor more than 90 days' notice of such redemption, at a redemption price equal to the principal amount plus accrued interest, if any, to the date fixed for redemption (except in the case of discounted debt securities, which may be redeemed at the redemption price specified by the terms of each series of such debt securities), if:

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        No notice of any such redemption may be given earlier than 90 days prior to the earliest date on which Aon plc would be obligated to pay any additional amounts.

        In the case of Aon plc debt securities and Aon Delaware debt securities benefiting from an Aon Delaware debt guarantee, Aon plc will also pay to each holder, or make available for payment to each such holder, on the redemption date, any additional amounts resulting from the payment of such redemption price by Aon plc as issuer or guarantor, as the case may be. Prior to the delivery of any notice of redemption, the issuer will deliver to the trustee an officer's certificate stating that it is entitled to effect or cause a redemption and setting forth a statement of facts showing that the conditions precedent of the right so to redeem or cause such redemption have occurred, and in the case of a redemption based on an opinion of independent counsel referred to in the second bullet above, such independent counsel's opinion.

        Any notice of redemption will be irrevocable once an officer's certificate has been delivered to the trustee.

Defeasance

        Defeasance and Discharge.    Unless the debt securities of any series provide otherwise, the issuer and, if applicable, the guarantor may be discharged from any and all obligations in respect of the debt securities of that series and any related guarantee, as applicable (except for certain obligations to register the transfer or exchange of debt securities of that series, to replace stolen, destroyed, lost or mutilated debt securities of that series, to maintain paying agencies, to execute and furnish definitive securities evidenced by temporary securities, to return moneys deposited with or paid to the trustee or any paying agent remaining unclaimed for three years, to compensate and indemnify the applicable trustee or to furnish such trustee (if that trustee is not the registrar) with the names and addresses of holders of debt securities of that series). This discharge, referred to as defeasance, will occur only if, among other things:

        That opinion must state that the issuer or, if applicable, the guarantor has received from the United States Internal Revenue Service a ruling or, since the date of execution of the applicable indenture, there has been a change in the applicable United States federal income tax law, in any case, in support of that opinion. (Sections 13.02 and 13.04 of the indentures)

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        In addition, the issuer or, if applicable, the guarantor or both may also obtain a discharge of either indenture with respect to all debt securities issued under that indenture and any related guarantee, as applicable, by depositing with the applicable trustee, in trust, enough money to pay all amounts due on the debt securities on the date those payments are due or upon redemption of all of those debt securities, so long as those debt securities are by their terms to become due and payable within one year or are to be called for redemption within one year. (Section 12.01 of the indentures)

        Defeasance of Certain Covenants and Certain Events of Default.    Unless the debt securities of any series provide otherwise, upon compliance with certain conditions:

        The conditions include, among other things:

        Covenant Defeasance and Certain Other Events of Default.    If the issuer or, if applicable, the guarantor exercises or both exercise the option to effect a covenant defeasance with respect to the debt securities of any series as described above and the debt securities of that series are thereafter declared due and payable because of an event of default (other than an event of default caused by failing to comply with the covenants that are defeased), the amount of money and securities it has or they have deposited with the applicable trustee would be sufficient to pay amounts due on the debt securities of that series on their respective due dates but may not be sufficient to pay amounts due on the debt

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securities of that series at the time of acceleration resulting from that event of default. However, the issuer and, if applicable, the guarantor would remain liable for any shortfall.

Modification and Waiver

        Each indenture provides that the issuer and, if applicable, the guarantor may enter into supplemental indentures with the applicable trustee without the consent of the holders of debt securities to:

        The issuer and, if applicable, the guarantor may enter into a supplemental indenture to modify an indenture with the consent of the applicable trustee and the holders of at least a majority in principal amount of outstanding debt securities of each series affected by such supplemental indenture. However, the issuer and, if applicable, the guarantor may not modify an indenture without the consent of the holders of all then-outstanding debt securities of the affected series issued under that indenture to:

        The holders of at least a majority in principal amount of the outstanding debt securities of any series may on behalf of the holders of all debt securities of that series waive any past default under the applicable indenture with respect to that series, except a default in the payment of the principal of or any premium or any interest on, any debt security of that series or in respect of a provision which under the applicable indenture cannot be modified or amended without the consent of the holder of each outstanding debt security of that affected series. (Section 6.09 of the indentures)

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Global Securities

        The debt securities of a series may be issued in whole or in part in the form of one or more global certificates that the issuer will deposit with a depositary identified in an applicable prospectus supplement. Unless and until it is exchanged in whole or in part for the individual debt securities that it represents, a global security may not be transferred except as a whole:

        An applicable prospectus supplement will describe the specific terms of the depositary arrangement with respect to a series of debt securities. We anticipate that the following provisions will generally apply to depositary arrangements.

        When a global security is issued, the depositary for the global security or its nominee will credit, on its book-entry registration and transfer system, the respective principal amounts of the individual debt securities represented by that global security to the accounts of persons that have accounts with the depositary ("participants"). Those accounts will be designated by the dealers, underwriters or agents with respect to the underlying debt securities or by the issuer if those debt securities are offered and sold directly by the issuer. Ownership of beneficial interests in a global security will be limited to participants or persons that may hold interests through participants. For interests of participants, ownership of beneficial interests in the global security will be shown on records maintained by the applicable depositary or its nominee. For interests of persons other than participants, that ownership information will be shown on the records of participants. Transfer of that ownership will be effected only through those records. The laws of some states require that certain purchasers of securities take physical delivery of securities in definitive form. These limits and laws may impair our ability to transfer beneficial interests in a global security.

        As long as the depositary for a global security, or its nominee, is the registered owner of that global security, the depositary or nominee will be considered the sole owner or holder of the debt securities represented by the global security for all purposes under the applicable indenture. Except as provided below, owners of beneficial interests in a global security:

        Payments of the principal of, any premium on and any interest on individual debt securities represented by a global security registered in the name of a depositary or its nominee will be made to the depositary or its nominee as the registered owner of the global security representing such debt securities. No issuer, guarantor, trustee, paying agent or registrar for the debt securities will be responsible for any aspect of the records relating to or payments made by the depositary or any participants on account of beneficial interests in the global security.

        It is expected that the depositary or its nominee, upon receipt of any payment of principal, any premium or interest relating to a global security representing any series of debt securities, immediately will credit participants' accounts with the payments. Those payments will be credited in amounts proportional to the respective beneficial interests of the participants in the principal amount of the global security as shown on the records of the depositary or its nominee. It is also expected that payments by participants to owners of beneficial interests in the global security held through those

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participants will be governed by standing instructions and customary practices. This is now the case with securities held for the accounts of customers registered in "street name." Those payments will be the sole responsibility of those participants.

        If the depositary for a series of debt securities is at any time unwilling, unable or ineligible to continue as depositary and a successor depositary is not appointed within 90 days, the issuer will issue individual debt securities of that series in exchange for the global security or securities representing that series. In addition, the issuer may at any time in its sole discretion determine not to have any debt securities of a series represented by one or more global securities. In that event, the issuer will issue individual debt securities of that series in exchange for the global security or securities. Furthermore, if specified in an applicable prospectus supplement, an owner of a beneficial interest in a global security may, on terms acceptable to the issuer, the trustee and the applicable depositary, receive individual debt securities of that series in exchange for those beneficial interests. The foregoing is subject to any limitations described in an applicable prospectus supplement. In any such instance, the owner of the beneficial interest will be entitled to physical delivery of individual debt securities equal in principal amount to the beneficial interest and to have the debt securities registered in its name. Those individual debt securities will be issued in any authorized denominations.

Subordination under the Aon plc Subordinated Debt Indenture

        The Aon plc subordinated debt securities and the Aon plc subordinated debt guarantees will be subordinate and junior in right of payment to all senior indebtedness of Aon plc and Aon Delaware, respectively, to the extent provided in the Aon plc subordinated debt indenture. (Sections 16.01 and 16.03 of the Aon plc subordinated debt indenture) Neither Aon plc, as issuer, nor Aon Delaware, as Aon plc subordinated debt guarantor, may make any payments on account of principal or any premium, redemption, interest or any other amount payable under the Aon plc subordinated debt securities or the Aon plc subordinated debt guarantees, as the case may be, at any time when it has defaulted with respect to payment of principal or any premium, interest, sinking fund or other payment due on its senior indebtedness. (Section 16.02 of the Aon plc subordinated debt indenture) If either Aon plc, as issuer, or Aon Delaware, as Aon plc subordinated debt guarantor, makes any payment described in the foregoing sentence before all of its senior indebtedness is paid in full, such payment or distribution will be applied to pay off the applicable senior indebtedness which remains unpaid. Subject to the condition that the senior indebtedness of Aon plc or Aon Delaware, as the case may be, is paid in full, if any such payments are made on the senior indebtedness of Aon plc or Aon Delaware, as the case may be, as described above, the holders of Aon plc subordinated debt securities or Aon plc subordinated debt guarantees will be subrogated to the rights of the senior debt security holders of Aon plc or Aon Delaware, as the case may be. (Section 16.03 of the Aon plc subordinated debt indenture)

        The Aon plc subordinated debt indenture defines the term "senior indebtedness" to mean:

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        "Senior indebtedness" does not include:

        Because of the subordination provisions described above, some of the general creditors of Aon plc or Aon Delaware, as the case may be, may recover proportionately more than holders of the Aon plc subordinated debt securities or Aon plc subordinated debt guarantees if the assets of Aon plc or Aon Delaware, as the case may be, are distributed as a result of insolvency or bankruptcy. The Aon plc subordinated debt indenture provides that the subordination provisions will not apply to cash, properties and securities held in trust pursuant to the satisfaction and discharge and the legal defeasance provisions of the Aon plc subordinated debt indenture. (Section 16.03 of the Aon plc subordinated debt indenture) See "—Defeasance" for additional information regarding the legal defeasance provisions affecting the subordinated debt.

        We will set forth (or incorporate by reference) the approximate amount of senior indebtedness outstanding for each of Aon plc and Aon Delaware as of a recent date in any prospectus supplement under which we offer to sell Aon plc subordinated debt securities.

Subordination under the Aon Delaware Subordinated Debt Indenture

        The Aon Delaware subordinated debt securities and the Aon Delaware subordinated debt guarantees will be subordinate and junior in right of payment to all senior indebtedness of Aon Delaware and Aon plc, respectively, to the extent provided in the Aon Delaware subordinated debt indenture. (Sections 16.01 and 16.03 of the Aon Delaware subordinated debt indenture) Neither Aon Delaware, as issuer, nor Aon plc, as Aon Delaware subordinated debt guarantor, may make any payments on account of principal or any premium, redemption, interest or any other amount payable under the Aon Delaware subordinated debt securities or the Aon Delaware subordinated debt guarantees, as the case may be, at any time when it has defaulted with respect to payment of principal or any premium, interest, sinking fund or other payment due on its senior indebtedness. (Section 16.02 of the Aon Delaware subordinated debt indenture) If either Aon Delaware, as issuer, or Aon plc, as Aon Delaware subordinated debt guarantor, makes any payment described in the foregoing sentence before all of its senior indebtedness is paid in full, such payment or distribution will be applied to pay off the applicable senior indebtedness which remains unpaid. Subject to the condition that the senior indebtedness of Aon Delaware or Aon plc, as the case may be, is paid in full, if any such payments are made on the senior indebtedness of Aon Delaware or Aon plc, as the case may be, as described above, the holders of Aon Delaware subordinated debt securities or Aon Delaware subordinated debt guarantees will be subrogated to the rights of the senior debt security holders of Aon Delaware or Aon plc, as the case may be. (Section 16.03 of the Aon Delaware subordinated debt indenture)

        The Aon Delaware subordinated debt indenture defines the term "senior indebtedness" to mean:

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        "Senior indebtedness" does not include:

        Because of the subordination provisions described above, some of the general creditors of Aon Delaware or Aon plc, as the case may be, may recover proportionately more than holders of the Aon Delaware subordinated debt securities or Aon Delaware subordinated debt guarantees if the assets of Aon plc or Aon Delaware, as the case may be, are distributed as a result of insolvency or bankruptcy. The Aon Delaware subordinated debt indenture provides that the subordination provisions will not apply to cash, properties and securities held in trust pursuant to the satisfaction and discharge and the legal defeasance provisions of the Aon Delaware subordinated debt indenture. (Section 16.03 of the Aon Delaware subordinated debt indenture) See "—Defeasance" for additional information regarding the legal defeasance provisions affecting the subordinated debt.

        We will set forth (or incorporate by reference) the approximate amount of senior indebtedness outstanding for each of Aon Delaware and Aon plc as of a recent date in any prospectus supplement under which we offer to sell Aon Delaware subordinated debt securities.

Guarantees

        Under each guarantee, the applicable guarantor will unconditionally guarantee the due and punctual payment of the principal, interest (if any), premium (if any) and all other amounts due on the applicable debt securities when the same shall become due and payable, whether at maturity, pursuant to mandatory or optional redemption or repayments, by acceleration or otherwise, in each case after any applicable grace periods or notice requirements, according to the terms of the applicable debt securities.

        The obligations of each guarantor under the guarantees will be unconditional, regardless of the enforceability of the applicable debt securities, and will not be discharged until all obligations under those debt securities and the applicable indenture are satisfied. Holders of the applicable debt securities may proceed directly against the guarantor under the applicable guarantee if an event of default affecting those debt securities occurs without first proceeding against the issuer.

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Conversion Rights

        An applicable prospectus supplement will describe the terms and conditions, if any, on which debt securities being offered are convertible into our Class A Ordinary Shares or other securities. Such terms will include the conversion price, the conversion period, provisions as to whether conversion will be at the issuer's option or the option of the holder, the events requiring an adjustment of the conversion price and provisions affecting conversion in the event of the redemption of those debt securities.

Regarding the Trustee

        The issuers have commercial deposits and custodial arrangements with The Bank of New York—Mellon Trust Company, N.A., or "BNYM," and may have borrowed money from BNYM in the normal course of business. The issuers may enter into similar or other banking relationships with BNYM in the future in the normal course of business. In addition, the issuers have provided brokerage and other insurance services in the ordinary course of their respective businesses for BNYM. BNYM may also act as trustee with respect to other debt securities one or both of the issuers have issued.

        BNYM will be serving as the trustee under the senior indentures and the subordinated indentures. Consequently, if an actual or potential event of default occurs with respect to either the senior debt securities or the subordinated debt securities, BNYM may be considered to have a conflicting interest for purposes of the Trust Indenture Act. In that case, BNYM may be required to resign under one or more indentures, and the applicable issuer and, if applicable, the applicable guarantor would be required to appoint a successor trustee. For this purpose, a "potential" event of default means an event that would be an event of default if the requirements for giving us default notice or for the default having to exist for a specific period of time were disregarded.

Governing Law

        The debt securities, the guarantees and the indentures will be governed by and construed in accordance with the laws of the State of New York, except that, as the indentures specify, the subordination provisions of each series of Aon plc subordinated debt securities, any Aon Delaware subordinated debt guarantee and the Aon plc subordinated indenture, as they apply to Aon plc, will be governed by and construed in accordance with the laws of England and Wales (other than the appointment of any attorney-in-fact, which shall be governed by the laws of the State of New York).


DESCRIPTION OF PREFERENCE SHARES

        In this description, references to "Aon," the "company," "we," "us" or "our" refer only to Aon plc and not to any of our subsidiaries or affiliates. Also, in this section, references to "holders" mean those who own preference shares registered in their own names, on the books that our registrar or we maintain for this purpose, and not those who own beneficial interests in preference shares registered in "street name" or in shares issued in book-entry form and held through one or more depositaries.

        The description set forth below is only a summary and is not complete. For more information regarding the preference shares which may be offered by this prospectus, please refer to the documents incorporated by reference in this prospectus, the applicable prospectus supplement or supplements, our articles of association ("Articles"), which are incorporated by reference as an exhibit to the registration statement of which this prospectus forms a part, and any certificate of designations or other instrument establishing a series of preference shares, which will be filed with the SEC as an exhibit to or incorporated by reference in the registration statement at or prior to the time of the issuance of that series of preference shares.

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        Our Articles authorize us to issue preference shares, in one or more series, with the number of shares of each series and the rights, preferences and limitations of each series to be determined by or pursuant to the authorization of our board of directors at the time of issue of the relevant preference shares. Our board of directors has been authorized to allot and issue up to 25,000,000 preference shares all of which remain authorized for allotment and issuance. This authority will expire on March 29, 2017, which is the date five years from the date we adopted our current Articles. Our preference shares will likely be afforded preferences regarding dividends and liquidation rights over our Class A Ordinary Shares and may be issued as redeemable shares, with all such other rights, terms and conditions to be determined by or pursuant to the authorization of the board of directors of Aon plc at the time of issue of the relevant preference shares.

        We will include the specific terms of each series of the preference shares being offered in an applicable prospectus supplement.


DESCRIPTION OF CLASS A ORDINARY SHARES

        In this description, references to "Aon," the "company," "we," "us" or "our" refer only to Aon plc and not to any of our subsidiaries or affiliates. Also, in this section, references to "holders" mean those who own ordinary shares of Aon registered in their own names, on the books that the registrar or we maintain for this purpose, and not those who own beneficial interests in ordinary shares registered in street name or in shares issued in book-entry form and held through one or more depositaries.

        The description of our Class A Ordinary Shares is incorporated into this prospectus by reference to the Current Report on Form 8-K12B filed with the SEC on April 2, 2012. Our Class A Ordinary Shares are listed on the New York Stock Exchange under the symbol "AON."

        For more information regarding the Class A Ordinary Shares which may be offered by this prospectus, please refer to the applicable prospectus supplement or supplements, our Articles, which are incorporated by reference as an exhibit to the registration statement of which this prospectus forms a part, and any other instrument relating to our ordinary shares filed with the SEC as an exhibit to or incorporated by reference in the registration statement at or prior to the time of the issuance of our Class A Ordinary Shares.

        Our Articles authorize us to allot and issue Class A Ordinary Shares, and to grant rights to subscribe for or to convert or exchange any security into or for Class A Ordinary Shares, which shall have the same rights, preferences and limitations as the existing Class A Ordinary Shares. Our board has been authorized to issue up to 415,000,000 Class A Ordinary Shares in addition to the Class A Ordinary Shares issued in connection with the Redomestication. Of this amount, 412,365,479 shares remain authorized for allotment and issuance. This authority will expire on March 29, 2017, which is the date five years from the date we adopted our current Articles.


DESCRIPTION OF SHARE PURCHASE CONTRACTS
AND SHARE PURCHASE UNITS

        In this description, references to "Aon," the "company," "we," "us" or "our" refer only to Aon plc and not to any of our subsidiaries. We may issue share purchase contracts, representing contracts obligating holders to purchase from us, and obligating us to sell to the holders, a specified number of our Class A Ordinary Shares at a future date or dates. The price per share and the number of our Class A Ordinary Shares may be fixed at the time the share purchase contracts are issued or may be determined by reference to a specific formula set forth in the share purchase contracts. The share purchase contracts may be issued separately or as a part of share purchase units consisting of a share

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purchase contract and, as security for the holder's obligations to purchase our Class A Ordinary Shares under our Class A Ordinary Shares purchase contracts:

        The share purchase contracts may require us to make periodic payments to the holders of the share purchase units or vice versa, and such payments may be unsecured or prefunded on some basis. The share purchase contracts may require holders to secure their obligations in a specified manner and, in certain circumstances, we may deliver newly issued prepaid share purchase contracts upon release to a holder of any collateral securing such holder's obligations under the original share purchase contract.

        An applicable prospectus supplement will describe the terms of any share purchase contracts or share purchase units and, if applicable, prepaid share purchase contracts.


PLAN OF DISTRIBUTION

        We or Aon Delaware may sell the securities covered by this prospectus in any of the following ways (or in any combination):

        Any such dealer or agent, in addition to any underwriter, may be deemed to be an underwriter within the meaning of the Securities Act of 1933, as amended (the "Securities Act"). Any discounts or commissions received by an underwriter, dealer, remarketing firm or agent on the sale or resale of securities may be considered by the SEC to be underwriting discounts and commissions under the Securities Act.

        In addition, we or Aon Delaware may enter into derivative transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If an applicable prospectus supplement so indicates, in connection with such a transaction, the third parties may, pursuant to this prospectus and that applicable prospectus supplement, sell securities covered by this prospectus and that applicable prospectus supplement. If so, the third party may use securities borrowed from us or Aon Delaware or others to settle such sales and may use securities received from us to close any related short positions. We or Aon Delaware may also loan or pledge securities covered by this prospectus and an applicable prospectus supplement to third parties, who may sell the loaned securities or, in an event of default in the case of a pledge, sell the pledged securities covered by this prospectus and that applicable prospectus supplement.

        The terms of the offering of the securities with respect to which this prospectus is being delivered will be set forth in the applicable prospectus supplement or supplements and will include, among other things:

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        If underwriters are used in the sale of securities, those securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The securities may be offered to the public either through underwriting syndicates represented by managing underwriters or directly by one or more underwriters acting alone. Unless otherwise set forth in an applicable prospectus supplement, the obligations of the underwriters to purchase the securities described in an applicable prospectus supplement will be subject to certain conditions precedent, and the underwriters will be obligated to purchase all of those securities if any are purchased by them. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

        If the dealers acting as principals are used in the sale of any securities, those securities will be acquired by the dealers, as principals, and may be resold from time to time in one or more transactions at varying prices to be determined by the dealer at the time of resale. The name of any dealer and the terms of the transactions will be set forth in an applicable prospectus supplement with respect to the securities being offered.

        Securities may also be offered and sold, if so indicated in an applicable prospectus supplement in connection with a remarketing upon their purchase, in accordance with a redemption or repayment pursuant to their terms, or otherwise, by one or more firms, which we refer to as the "remarketing firms," acting as principals for their own accounts or as our agents, as applicable. Any remarketing firm will be identified and the terms of its agreement, if any, with us or Aon Delaware and its compensation will be described in an applicable prospectus supplement. Remarketing firms may be deemed to be underwriters, as that term is defined in the Securities Act in connection with the securities remarketed thereby.

        The securities may be sold directly by us or Aon Delaware or through agents designated by us from time to time. In the case of securities sold directly by us or Aon Delaware, no underwriters or agents would be involved. Any agents involved in the offer or sale of the securities in respect of which this prospectus is being delivered, and any commissions payable by us or Aon Delaware to such agents, will be set forth in an applicable prospectus supplement. Unless otherwise indicated in an applicable prospectus supplement, any such agent will be acting on a best efforts basis for the period of its appointment.

        We or Aon Delaware may authorize agents, underwriters or dealers to solicit offers by certain specified institutions to purchase the securities to which this prospectus and the applicable prospectus supplement or supplements relate from us at the public offering price set forth in an applicable prospectus supplement plus, if applicable, accrued interest, pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. Those contracts will be subject only to those conditions set forth in an applicable prospectus supplement, and an applicable prospectus supplement will set forth the commission payable for solicitation of those contracts.

        Agents, dealers, underwriters and remarketing firms may be entitled, under agreements entered into with us, Aon Delaware or both, to indemnification by us, Aon Delaware or both against certain civil liabilities, including liabilities under the Securities Act, or to contribution to payments they may be

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required to make in respect thereof. Agents, dealers, underwriters and remarketing firms may be customers of, engage in transactions with, or perform services for us or our subsidiaries in the ordinary course of business.

        Unless otherwise indicated in an applicable prospectus supplement, all securities offered by this prospectus, other than our Class A Ordinary Shares, which are listed on the New York Stock Exchange, will be new issues with no established trading market. We or Aon Delaware, as applicable, may elect to list any of the securities on one or more exchanges, but unless otherwise specified in an applicable prospectus supplement, neither we nor Aon Delaware shall be obligated to do so. In addition, underwriters will not be obligated to make a market in any securities. No assurance can be given regarding the activity of trading in, or liquidity of, any securities.

        Any underwriter may engage in over-allotment, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Over-allotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying securities so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.


VALIDITY OF SECURITIES

        Sidley Austin LLP will be requested to advise us with respect to the validity under New York law of any securities that may be offered pursuant to this prospectus and one or more applicable prospectus supplements. Freshfields Bruckhaus Deringer LLP will be requested to advise us with respect to the validity under English law of any securities that may be offered pursuant to this prospectus and one or more applicable prospectus supplements.


EXPERTS

        The consolidated financial statements of Aon Corporation, as predecessor to Aon plc, incorporated by reference in Aon's Annual Report (Form 10-K) for the year ended December 31, 2011, and the effectiveness of Aon Corporation's internal control over financial reporting as of December 31, 2011 have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, incorporated by reference therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

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$                        

LOGO

Aon plc

        % Senior Notes due 20      
With a full and unconditional guarantee as to payment of
principal and interest by Aon Corporation



PROSPECTUS SUPPLEMENT



Morgan Stanley
Barclays
HSBC
J.P. Morgan

May     , 2015