PROSPECTUS SUPPLEMENT
(To Prospectus Dated March 6, 1998)

                               OMNICOM GROUP INC.
                        $230,000,000 Principal Amount of
               2 1/4% Convertible Subordinated Debentures due 2013
                     (Interest Payable January 6 and July 6)
                        4,615,694 Shares of Common Stock

                             ---------------------

      This document  supplements the Prospectus  dated March 6, 1998 relating to
(i) $230,000,000  aggregate principal amount of 2 1/4% Convertible  Subordinated
Debentures  due 2013  (the  "Debentures")  of  Omnicom  Group  Inc.,  a New York
corporation  ("Omnicom" or the "Company"),  and (ii) 4,615,694  shares of Common
Stock,  par value $0.50 per share (the "Common  Stock") of the Company which are
initially   issuable  upon   conversion  of  the  Debentures   plus   additional
indeterminate  number  of shares of Common  Stock as may  become  issuable  upon
conversion of the Debentures as a result of adjustments to the conversion  price
(the  "Shares").  The  Debentures  were  initially  acquired from the Company by
Morgan Stanley & Co.  Incorporated  in January 1998 in connection with a private
offering.  This  Prospectus  Supplement is  incorporated  by reference  into the
Prospectus, and all terms used herein shall have the meaning assigned to them in
the  Prospectus.  On May 27,  1998  the  closing  price of the  Common  Stock as
reported on the New York Stock Exchange was $46.875 per share.  The Common Stock
is traded under the symbol "OMC".

      In  accordance  with  the  Section  of the  Prospectus  entitled  "Selling
Securityholders"  (which  appears on page 19 of the  Prospectus),  the following
information is provided with respect to the beneficial owners of the Debentures:

                                          Principal Amount     Principal Amount
                                           of Debentures     of Debentures to be
Name of Selling Securityholder(s):       Beneficially Owned    Offered for Sale
- ---------------------------------        ------------------  -------------------
Amerisure Companies/Michigan
  Mutual Insurance Company                   $  180,000           $  180,000
Donaldson, Lufkin & Jenrettte
  Securities Corp.                              225,000              225,000
Jefferies & Company                           1,000,000            1,000,000
LB Series Fund, Inc. - 
  Income Portfolio                              850,000              850,000
Lord Abbett & Co. for 
  ILA Annuity & Co.                           1,000,000            1,000,000
Lutheran Brotherhood                          4,000,000            4,000,000
Lutheran Brotherhood Income Fund                600,000              600,000
The Travelers Indemnity Company(1)            2,800,000            2,800,000
The Travelers Insurance Company(2)            1,800,000            1,800,000
The Travelers Life & Annuity 
  Company(3)                                    200,000              200,000
Travelers Managed Assets Trust                  200,000              200,000

- ----------
(1) Holds $2,858,000 principal amount of registered Debentures.
(2) Holds $1,827,000 principal amount of registered Debentures.
(3) Holds $215,000 principal amount of registered Debentures.

      Other  than as set forth in the  footnotes  above,  the  Debentures  being
offered by the Selling  Securityholders  hereby  represent all of the Debentures
beneficially owned by the Selling Securityholders as of May 27, 1998. Except for
purchase of the Debentures and the purchase of the Company's 4 1/4%  Convertible
Subordinated  Debentures  due  2007  by The  Travelers  Indemnity  Company,  The
Travelers Insurance Company,  The Travelers Life & Annuity Company and Travelers
Managed  Assets Trust,  none of the Selling  Securityholders  has had a material
relationship  with the  Company or any of its  affiliates  within the past three
years.

                            -----------------------

      THESE  SECURITIES  HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
AND EXCHANGE  COMMISSION OR ANY STATE SECURITY COMMISSION NOR HAS THE SECURITIES
AND  EXCHANGE  COMMISSION  OR ANY STATE  SECURITIES  COMMISSION  PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

                            -----------------------

            The date of this Prospectus Supplement is May 29, 1998.