<PAGE>   1
                                                                      Exhibit 24


                                 OM Group, Inc.

                                    Form 10-K
                         Power of Attorney for Directors


The undersigned, a director of OM Group, Inc., a Delaware corporation (the
"Company"), which anticipates the filing with the Securities and Exchange
Commission (the "Commission") under the provisions of the Securities Exchange
Act of 1934 (the "Act") a Form 10-K (together with any and all subsequent
amendments, the "Form 10-K"), does hereby constitute and appoint James P.
Mooney, James M. Materna or Michael J. Scott and any one of them with full power
of substitution and resubstitution, as attorney or attorneys to execute and file
on behalf of the undersigned, in his capacity as director of the Company, the
Form 10-K and any and all other documents to be filed with the Commission
pertaining to the Form 10-K, with full power and authority to do and perform any
and all acts and things whatsoever required or necessary to be done in the
premises, as fully as to all intents and purposes as he could do if personally
present, hereby ratifying and approving the acts of said attorneys and any of
them and any such substitution.

Executed this 22nd day of March, 1999.


                                                                  /s/ Eugene Bak



<PAGE>   2


                                 OM Group, Inc.

                                    Form 10-K
                         Power of Attorney for Directors


The undersigned, a director of OM Group, Inc., a Delaware corporation (the
"Company"), which anticipates the filing with the Securities and Exchange
Commission (the "Commission") under the provisions of the Securities Exchange
Act of 1934 (the "Act") a Form 10-K (together with any and all subsequent
amendments, the "Form 10-K"), does hereby constitute and appoint James P.
Mooney, James M. Materna or Michael J. Scott and any one of them with full power
of substitution and resubstitution, as attorney or attorneys to execute and file
on behalf of the undersigned, in his capacity as director of the Company, the
Form 10-K and any and all other documents to be filed with the Commission
pertaining to the Form 10-K, with full power and authority to do and perform any
and all acts and things whatsoever required or necessary to be done in the
premises, as fully as to all intents and purposes as he could do if personally
present, hereby ratifying and approving the acts of said attorneys and any of
them and any such substitution.

Executed this 22nd day of March, 1999.


                                                              /s/ Lee R. Brodeur



<PAGE>   3


                                 OM Group, Inc.

                                    Form 10-K
                         Power of Attorney for Directors


The undersigned, a director of OM Group, Inc., a Delaware corporation (the
"Company"), which anticipates the filing with the Securities and Exchange
Commission (the "Commission") under the provisions of the Securities Exchange
Act of 1934 (the "Act") a Form 10-K (together with any and all subsequent
amendments, the "Form 10-K"), does hereby constitute and appoint James P.
Mooney, James M. Materna or Michael J. Scott and any one of them with full power
of substitution and resubstitution, as attorney or attorneys to execute and file
on behalf of the undersigned, in his capacity as director of the Company, the
Form 10-K and any and all other documents to be filed with the Commission
pertaining to the Form 10-K, with full power and authority to do and perform any
and all acts and things whatsoever required or necessary to be done in the
premises, as fully as to all intents and purposes as he could do if personally
present, hereby ratifying and approving the acts of said attorneys and any of
them and any such substitution.

Executed this 22nd day of March, 1999.


                                                                /s/ Frank Butler




<PAGE>   4


                                 OM Group, Inc.

                                    Form 10-K
                         Power of Attorney for Directors


The undersigned, a director of OM Group, Inc., a Delaware corporation (the
"Company"), which anticipates the filing with the Securities and Exchange
Commission (the "Commission") under the provisions of the Securities Exchange
Act of 1934 (the "Act") a Form 10-K (together with any and all subsequent
amendments, the "Form 10-K"), does hereby constitute and appoint James P.
Mooney, James M. Materna or Michael J. Scott and any one of them with full power
of substitution and resubstitution, as attorney or attorneys to execute and file
on behalf of the undersigned, in his capacity as director of the Company, the
Form 10-K and any and all other documents to be filed with the Commission
pertaining to the Form 10-K, with full power and authority to do and perform any
and all acts and things whatsoever required or necessary to be done in the
premises, as fully as to all intents and purposes as he could do if personally
present, hereby ratifying and approving the acts of said attorneys and any of
them and any such substitution.

Executed this 22nd day of March, 1999.


                                                           /s/ Thomas R. Miklich




<PAGE>   5


                                 OM Group, Inc.

                                    Form 10-K
                         Power of Attorney for Directors


The undersigned, a director of OM Group, Inc., a Delaware corporation (the
"Company"), which anticipates the filing with the Securities and Exchange
Commission (the "Commission") under the provisions of the Securities Exchange
Act of 1934 (the "Act") a Form 10-K (together with any and all subsequent
amendments, the "Form 10-K"), does hereby constitute and appoint James P.
Mooney, James M. Materna or Michael J. Scott and any one of them with full power
of substitution and resubstitution, as attorney or attorneys to execute and file
on behalf of the undersigned, in his capacity as director of the Company, the
Form 10-K and any and all other documents to be filed with the Commission
pertaining to the Form 10-K, with full power and authority to do and perform any
and all acts and things whatsoever required or necessary to be done in the
premises, as fully as to all intents and purposes as he could do if personally
present, hereby ratifying and approving the acts of said attorneys and any of
them and any such substitution.

Executed this 22nd day of March, 1999.



                                                             /s/ John E. Mooney





<PAGE>   6


                                 OM Group, Inc.

                                    Form 10-K
                         Power of Attorney for Directors


The undersigned, a director of OM Group, Inc., a Delaware corporation (the
"Company"), which anticipates the filing with the Securities and Exchange
Commission (the "Commission") under the provisions of the Securities Exchange
Act of 1934 (the "Act") a Form 10-K (together with any and all subsequent
amendments, the "Form 10-K"), does hereby constitute and appoint James P.
Mooney, James M. Materna or Michael J. Scott and any one of them with full power
of substitution and resubstitution, as attorney or attorneys to execute and file
on behalf of the undersigned, in his capacity as director of the Company, the
Form 10-K and any and all other documents to be filed with the Commission
pertaining to the Form 10-K, with full power and authority to do and perform any
and all acts and things whatsoever required or necessary to be done in the
premises, as fully as to all intents and purposes as he could do if personally
present, hereby ratifying and approving the acts of said attorneys and any of
them and any such substitution.

Executed this 22nd day of March, 1999.



                                                             /s/ Markku Toivanen