UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Mark One)

For the fiscal year ended December 31, 2004

OR

For the transition period from                               to                

Commission file number 001-12515

OM GROUP, INC.

(Exact name of Registrant as specified in its charter)

216-781-0083

Registrant’s telephone number, including area code

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act:

None

  Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes o   No x

  Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of the Registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.     o

  Indicate by check mark if the registrant is an accelerated filer (as defined in Rule 12b-2 of the Act). Yes x   No o

  Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes o   No x

  The aggregate market value of Common Stock, par value $.01 per share, held by nonaffiliates (based upon the closing sale price on the NYSE) on June 30, 2005 and June 30, 2004 was approximately $704 million and $940 million, respectively.

  As of June 30, 2005 there were 28,581,041 shares of Common Stock, par value $.01 per share, outstanding.

This amended Annual Report on Form 10-K/A is being filed solely for the purpose of including a conformed signature on the consent letter of Ernst & Young LLP in Exhibit 23.

Signature

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this Annual Report on Form 10-K/A to be signed on its behalf by the undersigned, thereunto duly authorized, on August 23, 2005.