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                                                Filed Pursuant to Rule 424(b)(3)
                                                      Registration No. 333-65624

PROSPECTUS

                                2,000,000 SHARES

                                AON CORPORATION

                                  COMMON STOCK
                                ---------------

    This prospectus relates to 2,000,000 shares of common stock of Aon
Corporation. The stockholders selling the shares in this offering have the right
to determine both the number of shares they will offer and the time or times
when they will offer the shares. They may sell the shares at the market price at
the time of sale or at such other prices as they may negotiate. All of the
shares covered by this prospectus were originally issued by Aon in connection
with the acquisition of First Extended, Inc.

    The selling stockholders will receive all of the net proceeds from the sale
of the shares offered hereby.

    The common stock is traded on the New York Stock Exchange under the symbol
"AOC."

    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.

    The information in this prospectus is not complete and may be changed. The
selling shareholders may not sell these securities until the registration
statement filed with the Securities and Exchange Commission is effective. This
prospectus is not an offer to sell these securities and it is not soliciting an
offer to buy these securities in any state where the offer or sale is not
permitted.

                 The date of this prospectus is August 9, 2001.
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                               TABLE OF CONTENTS

<Table>
<Caption>
                                                                PAGE
                                                              --------
                                                           
WHERE YOU CAN FIND MORE INFORMATION.........................     1

SPECIAL NOTE ON FORWARD-LOOKING STATEMENTS..................     2

PROSPECTUS SUMMARY..........................................     2

THE OFFERING................................................     3

USE OF PROCEEDS.............................................     3

SELLING STOCKHOLDERS........................................     3

PLAN OF DISTRIBUTION........................................     4

VALIDITY OF SECURITIES......................................     5

EXPERTS.....................................................     5
</Table>

                      WHERE YOU CAN FIND MORE INFORMATION

    We file annual, quarterly and special reports, proxy statements, and other
information with the SEC. You may read and copy any document we file with the
Securities and Exchange Commission at the SEC's Public Reference Room at 450
Fifth Street, N.W., Washington, D.C. 20549 or at the SEC's regional offices
located at Citicorp Center, 500 West Madison Street, Suite 1400, Chicago,
Illinois 60661-2511 and 7 World Trade Center, Suite 1300, New York, New York
10048. You may obtain further information on the operation of the Public
Reference Room by calling the SEC at 1-800-SEC-0330. Our SEC filings are also
available to the public over the Internet at the SEC's Web site at
http://www.sec.gov. 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, the
Chicago Stock Exchange, 440 South LaSalle Street, Chicago, Illinois 60605 and
the London Stock Exchange, Old Broad Street, London, England EC2N1HP. You may
find additional information about Aon Corporation and its subsidiaries at our
Web site at http://www.aon.com.

    This prospectus constitutes a part of a registration statement on Form S-3
filed by us with the SEC relating to the securities. As permitted by SEC rules,
this prospectus does not contain all of the information included in the
registration statement and the accompanying exhibits and schedules we file with
the SEC. You may refer to the registration statement and the exhibits and
schedules for more information about us and our securities. The registration
statement and exhibits and schedules are also available at the SEC's Public
Reference Room or through its Web site.

    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
referenced 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. We incorporate by reference the documents listed
below and any future filings we make with the SEC under Sections 13(a), 13(c),
14 or 15(d) of the Securities Exchange Act of 1934 until all of the securities
covered by this prospectus are sold by the selling stockholders.

    - Annual Report on Form 10-K for the year ended December 31, 2000;

    - Current Report on Form 8-K dated April 24, 2001;

    - Quarterly Report on Form 10-Q for the three months ended March 31, 2001;
      and

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    - The description of Aon Corporation's common stock contained in Item 12 of
      the registration statement on Form 10 filed on February 19, 1980 (when we
      were called Combined International Corporation), and any amendment or
      report which we have filed (or will file after the date of this prospectus
      and prior to the termination of this offering) for the purpose of updating
      such description, including Aon Corporation's Current Report on Form 8-K
      dated April 23, 1987 and Current Report on Form 8-K dated May 9, 2000.

    You may obtain a copy of these filings, at no cost, by writing to or
telephoning us at the following address:

                                Aon Corporation
                            200 East Randolph Street
                            Chicago, Illinois 60601
                            Telephone (312) 701-3000
                   Attention: Financial Relations Department

    You should rely only on the information incorporated by reference or
provided in this prospectus. We have not authorized anyone else to provide you
with different information. You should not assume that the information in this
prospectus is accurate as of any date other than the date on the cover of the
applicable document. We are not making an offer of the securities in any state
where the offer or sale is not permitted.

                   SPECIAL NOTE ON FORWARD-LOOKING STATEMENTS

    Some of the statements contained in or incorporated by reference in this
prospectus discuss our plans and strategies for our business or state other
forward-looking statements, as this term is defined in the Private Securities
Litigation Reform Act of 1995. The words "anticipates," "believes," "estimates,"
"expects," "plans," "intends" and similar expressions are intended to identify
these forward-looking statements, but are not the exclusive means of identifying
them. These forward-looking statements are subject to certain risks and
uncertainties that could cause actual results to differ materially from
historical results or those anticipated, depending on a variety of factors such
as general economic conditions in different countries around the world,
fluctuations in global equity and fixed income markets, changes in commercial
property and casualty premium rates, the competitive environment, the actual
cost of resolution of contingent liabilities, the final form of the business
transformation plan, the ultimate cost and timing of its implementation, the
actual costs, savings and other benefits resulting therefrom, whether Aon
ultimately implements the proposed spin-off of its underwriting operations and
the timing and terms associated therewith.

    In light of these risks, uncertainties and assumptions, the forward-looking
events discussed in or incorporated by reference in this prospectus might not
occur.

                               PROSPECTUS SUMMARY

AON CORPORATION

    Aon Corporation is an insurance services holding company for a family of
insurance brokerage, consulting and consumer insurance companies. Through its
insurance brokerage and other services and consulting operations, Aon offers
commercial insurance brokerage, alternative risk solutions, risk management,
employee benefit and human resources consulting and managing general
underwriting services. In addition, Aon Corporation's insurance underwriting
businesses provide a variety of insurance products, including accident and
health coverage, traditional life insurance and extended warranties. Aon's
revenues were $7.4 billion in 2000. Based on 2000 insurance brokerage and
consulting revenues, Aon believes that it is the second largest insurance
brokerage company in the world.

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    On April 20, 2001, Aon announced that its board of directors has approved,
in principle, a plan to spin off Aon's underwriting business to its common
stockholders, creating two independent, publicly-traded companies. The spin-off
would take the form of a tax-free stock dividend to Aon's common stockholders,
pending a favorable Internal Revenue Service ruling. The transaction requires
final board approval, and is subject to obtaining a favorable IRS ruling and
insurance regulatory approvals. For more information on the spin-off, please see
our Current Report on Form 8-K dated April 24, 2001.

ADDRESS

    Our executive offices are located at 200 East Randolph Street, Chicago,
Illinois 60601, Telephone (312) 701-3000.

                                  THE OFFERING

<Table>
                                            
Common stock offered by selling                2,000,000 shares
  stockholders...............................

Common stock outstanding as of May 31,         268,482,089 shares
  2001.......................................

New York Stock Exchange Symbol...............  AOC

Use of proceeds..............................  We will not receive any proceeds from the
                                               sale of the common stock being offered
                                               hereby.
</Table>

    The purpose of this offering is to register the resale of the shares of
common stock received by the selling stockholders in connection with our
acquisition of First Extended, Inc., a Delaware corporation. On July   , 2001,
Aon acquired all of the issued and outstanding common stock of First Extended,
through the merger of a wholly-owned subsidiary of Aon, with and into First
Extended. In connection with the merger, Aon entered into a registration rights
agreement, which requires Aon to use its best efforts to file, and cause to be
declared effective, a registration statement for the shares issued in the merger
and to keep the registration statement continuously effective until the earlier
of

    - the date on which all of the shares have been sold by the selling
      stockholders, and

    - two years following the date on which such registration statement was
      declared effective by the SEC.

                                USE OF PROCEEDS

    The selling stockholders will receive all of the proceeds from any sale of
our common stock offered under this prospectus. We will not receive any proceeds
from the sale of the common stock offered under this prospectus.

                              SELLING STOCKHOLDERS

    The following table sets forth:

    (1) the name of each selling stockholder,

    (2) the nature of any position, office or other material relationship which
       each selling stockholder has had with us or any of our affiliates within
       the last three years,

    (3) the number of shares of common stock owned by each selling stockholder
       prior to the offering,

    (4) the number of shares of common stock owned offered for each selling
       stockholder's account, and

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    (5) the number of shares of common stock owned by each such selling
       stockholder after completion of the offering.

    All of the selling stockholders were employees or stockholders of First
Extended at or prior to the date of the merger of a wholly-owned subsidiary of
Aon with and into First Extended.

<Table>
<Caption>
                                                NUMBER OF SHARES                      NUMBER OF SHARES
                                                  OWNED PRIOR      NUMBER OF SHARES     OWNED AFTER
SELLING STOCKHOLDER                               TO OFFERING         TO BE SOLD          OFFERING
- -------------------                             ----------------   ----------------   ----------------
                                                                             
Carl H. Westcott..............................     1,400,000          1,400,000              0
John D. Curtis................................       200,000            200,000              0
Charwes First Extended 1999 Trust.............       200,000            200,000              0
Courwes First Extended 1999 Trust.............       200,000            200,000              0
                                                   ---------          ---------
    Total.....................................     2,000,000          2,000,000
                                                   =========          =========
</Table>

                              PLAN OF DISTRIBUTION

    We will receive no part of the proceeds of any sales made hereunder. We will
pay all expenses of registration incurred in connection with this offering and
in connection with the offering and sale of the shares, including the reasonable
fees and expenses of one counsel selected by the selling stockholders to
represent the selling stockholders. All selling and other expenses incurred by
the selling stockholders will be borne by the selling stockholders, including
the fees and expenses of the selling stockholders' own counsel (other than the
counsel selected to represent all selling stockholders).

    The selling stockholders may from time to time sell all or a portion of the
shares on the New York Stock Exchange or on any national securities exchange on
which our common stock may be listed or traded, in negotiated transactions or
otherwise, at prices then prevailing or related to the then current market price
or at negotiated prices. The shares will not be sold in an underwritten public
offering. The shares may be sold directly or through brokers or dealers. The
methods by which the shares may be sold include:

    (1) a block trade (which may involve crosses) in which the broker or dealer
       so engaged will attempt to sell the shares as agent but may position and
       resell a portion of the block as principal to facilitate the transaction;

    (2) purchases by a broker or dealer as principal and resale by such broker
       or dealer for its account pursuant to this prospectus;

    (3) ordinary brokerage transactions and transactions in which the broker
       solicits purchasers; and

    (4) privately negotiated transactions.

    In effecting sales, brokers and dealers engaged by the selling stockholders
may arrange for other brokers or dealers to participate. Brokers or dealers may
receive commissions or discounts from the selling stockholders (or, if any such
broker-dealer acts as agent for the purchaser of such shares, from such
purchaser) in amounts to be negotiated which are not expected to exceed those
customary in the types of transactions involved. Broker-dealers may agree with
the selling stockholders to sell a specified number of such shares at a
stipulated price per share, and, to the extent such broker-dealer is unable to
do so acting as agent for a selling stockholder, to purchase as principal any
unsold shares at the price required to fulfill the broker-dealer commitment to
such selling stockholder. Broker-dealers who acquire shares as principal may
thereafter resell such shares from time to time in transactions (which may
involve crosses and block transactions and sales to and through other broker-
dealers, including transactions of the nature described above) in the over-the-
counter market or otherwise at prices and on terms then prevailing at the time
of sale, at prices then related to the then-current market price or

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in negotiated transactions and, in connection with such resales, may receive
from the purchasers of such shares commissions as described above.

    In connection with the distribution of the shares, the selling stockholders
may enter into hedging transactions with broker-dealers. In connection with such
transactions, broker-dealers may engage in short sales of the shares in the
course of hedging the positions they assume with the selling stockholders. The
selling stockholders may also sell the shares short and redeliver the shares to
close out the short positions. The selling stockholders may also enter into
option or other transactions with broker-dealers, which require the delivery to
the broker-dealer of the shares. The selling stockholders may also loan or
pledge the shares to a broker-dealer and the broker-dealer may sell the shares
so loaned or upon a default the broker-dealer may effect sales of the pledged
shares. In addition to the foregoing, the selling stockholders may enter into,
from time to time, other types of hedging transactions.

    The selling stockholders and any broker-dealers participating in the
distributions of the shares may be deemed to be "underwriters" within the
meaning of Section 2(11) of the Securities Act of 1933 and any profit on the
sale of shares by the selling stockholders and any commissions or discounts
given to any such broker-dealer may be deemed to be underwriting commissions or
discounts under that Act.

    In addition, any shares covered by this prospectus that qualify for sale
pursuant to Rule 144 or any other exemptions available under the Securities Act
may be sold under such exemptions rather than pursuant to this prospectus.

                             VALIDITY OF SECURITIES

    The validity of the securities will be passed upon for us by Richard E.
Barry, our Senior Counsel and Assistant Secretary. As of March 31, 2001,
Mr. Barry owned 4,059 shares of Aon Corporation common stock, held restricted
stock awards of 30,570 shares and 16,000 unvested stock options. In addition,
3,870 shares of Aon Corporation common stock held by its employee stock
ownership plan and savings plan were attributable to Mr. Barry.

                                    EXPERTS

    The consolidated financial statements of Aon Corporation incorporated by
reference in Aon Corporation's Annual Report (Form 10-K) for the year ended
December 31, 2000 and the related financial statement schedules included therein
have been audited by Ernst & Young LLP, independent auditors, as set forth in
their reports thereon included or incorporated by reference therein and
incorporated herein by reference. Such consolidated financial statements and
schedules are incorporated herein by reference in reliance upon such reports
given on the authority of such firm as experts in accounting and auditing.

    With respect to the unaudited condensed consolidated interim financial
information for the three-month periods ended March 31, 2001 and March 31, 2000
incorporated by reference in this prospectus, Ernst & Young LLP have reported
that they have applied limited procedures in accordance with professional
standards for a review of such information. However, their separate report,
included in Aon Corporation's Quarterly Report on Form 10-Q for the quarter
ended March 31, 2001 and incorporated herein by reference, states that they did
not audit and they do not express an opinion on that interim financial
information. Accordingly, the degree of reliance on their report on such
information should be restricted considering the limited nature of the review
procedures applied. The independent auditors are not subject to the liability
provisions of Section 11 of the Securities Act of 1933 (the "Act") for their
report on the unaudited interim financial information because that report is not
a "report" or a "part" of the Registration Statement prepared or certified by
the auditors within the meaning of Sections 7 and 11 of the Act.

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