Exhibit 24


                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS: That the undersigned has made, constituted
and appointed, and by this instrument does make, constitute and appoint ROBERT
PIERCE, ELIZABETH SHAUGHNESSY and CIPRIANO BEREDO, acting individually, as his
true and lawful attorney, for him, and in his name, place and stead, to affix,
as attorney-in-fact, the signature of the undersigned to any reports or filings
to the Securities and Exchange Commission on or in connection with Forms 3, 4, 5
or 144 with respect to transactions or holdings by the undersigned in equity
securities issued by OM Group, Inc., a Delaware corporation, and to any and all
amendments to such reports, giving and granting unto each such attorney-in-fact
full power and authority to do and perform every act and thing whatsoever
necessary to be done in the premises, as fully as the undersigned might or could
do if personally present, hereby ratifying and confirming all that each such
attorney-in-fact shall lawfully do or cause to be done by virtue hereof.

     This Power of Attorney shall expire on the date the undersigned is no
longer required to file Forms 3, 4, 5 or 144 reports with the Securities and
Exchange Commission with respect to holdings of and transactions in securities
issued by OM Group, Inc., unless revoked in writing prior thereto.

     IN WITNESS WHEREOF, this Power of Attorney has been signed in Cleveland,
Ohio, as of the 12th day of October, 2007.


                                        /s/ William J. Reidy
                                        ----------------------------------------
                                            William J. Reidy