Exhibit 32.1

                                CERTIFICATION OF
                                QUARTERLY REPORT
                                  ON FORM 10-Q

Pursuant  to 18  U.S.C.  ss.  1350,  as  adopted  pursuant  to  ss.  906  of the
Sarbanes-Oxley  Act of 2002,  in  connection  with the filing of  Omnicom  Group
Inc.'s  Quarterly  Report on Form 10-Q for the quarter ended September 30, 2003,
as filed with the  Securities  and Exchange  Commission  on the date hereof (the
"Report"), each of the undersigned officers of Omnicom certifies,  that, to such
officer's knowledge:

      o     the Report fully complies with the  requirements of Section 13(a) or
            15(d) of the Securities Exchange Act of 1934; and

      o     the  information  contained in the Report  fairly  presents,  in all
            material respects, the financial condition and results of operations
            of  Omnicom  as of the dates and for the  periods  expressed  in the
            Report.

Executed as of November 7, 2003.

                                                   /s/ John D. Wren
                                    --------------------------------------------
                                    Name:  John D. Wren
                                    Title: Chief Executive Officer and President

                                             /s/ Randall J. Weisenburger
                                    --------------------------------------------
                                    Name:  Randall J. Weisenburger
                                    Title: Executive Vice President and
                                           Chief Financial Officer