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 8, 1998 the closing price of the Common Stock as reported
on the New York Stock  Exchange  was  $48.3125  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 of        Principal Amount of
                                                       Debentures Beneficially        Debentures to be
      Name of Selling Shareholder(s)                             Owned                Offered for Sale
      ------------------------------                    ----------------------      -------------------
                                                                                           
      City of Birmingham Retirement & Relief System             2,100,000                  2,100,000
      Merrill Lynch Insurance Group                               300,000                    300,000
      Morgan Stanley Dean Witter                               16,175,000                 16,175,000
      Occidental Petroleum                                         50,000                     50,000
      Toronto Dominion (New York), Inc.                         5,000,000                  5,000,000
      TQA Arbitrage Fund, L.P.                                  1,000,000                  1,000,000


      The  Debentures  being  offered  by  the  Selling  Securityholders  hereby
represent   all  of  the   Debentures   beneficially   owned   by  the   Selling
Securityholders  as of May 8, 1998. Except for Morgan Stanley's  position as the
Initial  Purchaser  of  the  Debentures  and  as the  initial  purchaser  of the
Company's 4 1/4% Convertible  Subordinated Debentures due 2007 issued on January
3, 1997,  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.

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

      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 12, 1998.