PROSPECTUS SUPPLEMENT
(To Prospectus Dated March 11, 1997)

                               OMNICOM GROUP INC.
                        $218,500,000 Principal Amount of
               4 1/4% Convertible Subordinated Debentures due 2007
                     (Interest Payable January 3 and July 3)

                        6,936,508 Shares of Common Stock

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

      This document  supplements the Prospectus dated March 11, 1997 relating to
(i) $218,500,000  aggregate principal amount of 4 1/4% Convertible  Subordinated
Debentures  due 2007  (the  "Debentures")  of  Omnicom  Group  Inc.,  a New York
corporation  ("Omnicom" or the "Company"),  and (ii) 6,936,508  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 1997 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 22,  1998  the  closing  price of the  Common  Stock as
reported on the New York Stock Exchange was $47.625 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  pages 19 and 20 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
- ---------------------------------      --------------------   ------------------
Morgan Stanley Dean Witter                  $5,581,000            $5,581,000

      The  Debentures  being  offered  by  the  Selling  Securityholders  hereby
represent   all  of  the   Debentures   beneficially   owned   by  the   Selling
Securityholders  as of May 22, 1998. Except for Morgan Stanley's position as the
Initial  Purchaser of the  Debentures,  as initial  purchaser of the Company's 2
1/4% Convertible  Subordinated Debentures due 2013 issued on January 6, 1998 and
as lead  underwriter of 4,000,000 shares of the Company's Common Stock issued on
March 9, 1998 and except for the purchase of the Debentures, none of the Selling
Securityholders  has had a material  relationship with the Company or any of its
affiliates within the past three years.

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      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 27, 1998.