UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT

TO § 240.13d-1(a) AND AMENDMENTS THERETO FILED PURSUANT TO

§ 240.13d-2(a)

(Amendment No. 4)1

OM Group, Inc.

(Name of Issuer)

Common Stock, $0.01 par value per share

(Title of Class of Securities)

670872100

(CUSIP Number)

DAVID A. LORBER

FRONTFOUR CAPITAL GROUP LLC

35 Mason Street, 4th Floor

Greenwich, Connecticut 06830

(203) 274-9050

(Name, Address and Telephone Number of Person

Authorized to Receive Notices and Communications)

June 3, 2015

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box ¨.

Note:  Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits.  See § 240.13d-7 for other parties to whom copies are to be sent.

_______________

1              The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON FRONTFOUR CAPITAL GROUP LLC
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION DELAWARE
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 1,591,036
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 1,591,036
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,591,036
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 5.2%
14	TYPE OF REPORTING PERSON IA

2

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON FRONTFOUR MASTER FUND, LTD.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS WC
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION CAYMAN ISLANDS
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 1,018,113
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 1,018,113
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,018,113
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 3.3%
14	TYPE OF REPORTING PERSON CO

3

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON FRONTFOUR OPPORTUNITY FUND
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS WC
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION BRITISH COLUMBIA, CANADA
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 91,464
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 91,464
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 91,464
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) LESS THAN 1%
14	TYPE OF REPORTING PERSON OO

4

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON FRONTFOUR CAPITAL CORP.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION BRITISH COLUMBIA, CANADA
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 91,464
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 91,464
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 91,464
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) LESS THAN 1%
14	TYPE OF REPORTING PERSON CO

5

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON STEPHEN E. LOUKAS
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION USA
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 1,682,500
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 1,682,500
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,682,500
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 5.5%
14	TYPE OF REPORTING PERSON IN

6

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON DAVID A. LORBER
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION USA
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 1,682,500
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 1,682,500
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,682,500
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 5.5%
14	TYPE OF REPORTING PERSON IN

7

CUSIP NO. 670872100

1	NAME OF REPORTING PERSON ZACHARY R. GEORGE
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) o   (b) o
3	SEC USE ONLY
4	SOURCE OF FUNDS AF
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)		¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION USA
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	7	SOLE VOTING POWER - 0 -
	8	SHARED VOTING POWER 1,682,500
	9	SOLE DISPOSITIVE POWER - 0 -
	10	SHARED DISPOSITIVE POWER 1,682,500
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,682,500
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES		o
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 5.5%
14	TYPE OF REPORTING PERSON IN

8

CUSIP NO. 670872100

The following constitutes Amendment No. 4 to the Schedule 13D filed by the undersigned (“Amendment No. 4”).  This Amendment No. 4 amends the Schedule 13D as specifically set forth herein.

Item 3. Source and Amount of Funds or Other Consideration.

Item 3 is hereby amended and restated to read as follows:

The Shares and call options purchased by each of the Master Fund and the Canadian Fund, and the Shares and call options held in the Separately Managed Accounts, were purchased with working capital (which may, at any given time, include margin loans made by brokerage firms in the ordinary course of business). The aggregate purchase price of the 1,000,413 Shares owned directly by the Master Fund is approximately $27,192,596, including brokerage commissions.  The aggregate purchase price of the call options exercisable into 17,700 Shares owned directly by the Master Fund is approximately $11,543, including brokerage commissions.  The aggregate purchase price of the 89,864 Shares owned directly by the Canadian Fund is approximately $2,462,910, including brokerage commissions.  The aggregate purchase price of the call options exercisable into 1,600 Shares owned directly by the Canadian Fund is approximately $1,047, including brokerage commissions.  The aggregate purchase price of the 562,823 Shares held in the Separately Managed Accounts is approximately $15,412,378, including brokerage commissions.  The aggregate purchase price of the call options exercisable into 10,100 Shares held in the Separately Managed Accounts is approximately $7,123, including brokerage commissions.

Item 4. Purpose of Transaction.

Item 4 is hereby amended to add the following:

On June 3, 2015, David A. Lorber resigned from the Issuer’s Board of Directors (the “Board”) in light of the Issuer’s announced acquisition on June 1, 2015, the same date that Mr. Lorber was elected to the Board, by funds managed by affiliates of Apollo Global Management, LLC for $34.00 per Share in cash. Pursuant to the Settlement Agreement by and among FrontFour Capital and certain of its affiliates (“FrontFour”) and the Issuer, dated March 23, 2015, FrontFour has notified the Issuer that it intends to exercise its right to identify a replacement director, who shall qualify as “independent” pursuant to New York Stock Exchange listing standards and the Issuer’s Corporate Governance Principles, and have relevant financial and business experience, to fill the resulting vacancy on the Board.

Item 5. Interest in Securities of the Issuer.

Items 5(a) – (c) are hereby amended and restated to read as follows:

(a)           The aggregate percentage of Shares reported owned by each person named herein is based upon 30,738,083 Shares outstanding as of May 26, 2015, which is the total number of Shares outstanding as reported in Exhibit 2.1 to the Issuer’s Current Report on Form 8-K filed with the Securities and Exchange Commission on June 1, 2015.

As of the close of business on June 4, 2015, the Master Fund beneficially owned directly 1,018,113 Shares (including 17,700 Shares underlying certain call options exercisable within 60 days of the date hereof), the Canadian Fund beneficially owned directly 91,464 Shares (including 1,600 Shares underlying certain call options exercisable within 60 days of the date hereof), and 572,923 Shares (including 10,100 Shares underlying certain call options exercisable within 60 days of the date hereof) were held in the Separately Managed Accounts, constituting approximately 3.3%, less than 1%, and approximately 1.9%, respectively, of the Shares outstanding.

9

CUSIP NO. 670872100

FrontFour Capital, as the investment manager of the Master Fund and the Separately Managed Accounts, may be deemed to beneficially own the 1,591,036 Shares directly beneficially owned in the aggregate by the Master Fund and held in the Separately Managed Accounts, constituting approximately 5.2% of the Shares outstanding.

FrontFour Corp., as the investment manager of the Canadian Fund, may be deemed to beneficially own the 91,464 Shares directly beneficially owned by the Canadian Fund, constituting less than 1% of the Shares outstanding.

Each of Messrs. Loukas, Lorber and George, as a managing member and principal owner of FrontFour Capital and a principal owner of FrontFour Corp., may be deemed to beneficially own the 1,682,500 Shares directly beneficially owned in the aggregate by the Master Fund and the Canadian Fund and held in the Separately Managed Accounts, constituting approximately 5.5% of the Shares outstanding.

Each of the Reporting Persons, as a member of a “group” with the other Reporting Persons for purposes of Rule 13d-5(b)(1) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), may be deemed to beneficially own the securities of the Issuer owned by the other Reporting Persons.  The filing of this Schedule 13D shall not be deemed an admission that the Reporting Persons are, for purposes of Section 13(d) of the Exchange Act, the beneficial owners of any securities of the Issuer he or it does not directly own.  Each of the Reporting Persons specifically disclaims beneficial ownership of the securities reported herein that he or it does not directly own.

(b)           Each of the Master Fund, FrontFour Capital and Messrs. Loukas, Lorber and George have shared power to vote or direct the vote of, and to dispose or direct the disposition of, the Shares owned directly by the Master Fund.

Each of FrontFour Capital and Messrs. Loukas, Lorber and George have shared power to vote or direct the vote of, and to dispose or direct the disposition of, the Shares held in the Separately Managed Accounts.

Each of the Canadian Fund, FrontFour Corp. and Messrs. Loukas, Lorber and George have shared power to vote or direct the vote of, and to dispose or direct the disposition of, the Shares owned directly by the Canadian Fund.

(c)           The transactions in the securities of the Issuer by the Reporting Persons during the past sixty days are set forth in Schedule A and are incorporated herein by reference.

10

CUSIP NO. 670872100

SIGNATURES

After reasonable inquiry and to the best of his knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Dated: June 5, 2015	FrontFour Master Fund, Ltd.
	By:	FrontFour Capital Group LLC as Investment Manager
	By:	/s/ David A. Lorber
		Name:	David A. Lorber
		Title:	Managing Member

FrontFour Capital Group LLC
By:	/s/ David A. Lorber
	Name:	David A. Lorber
	Title:	Managing Member

FrontFour Opportunity Fund
By:	FrontFour Capital Corp. as Investment Manager
By:	/s/ David A. Lorber
	Name:	David A. Lorber
	Title:	Authorized Signatory

FrontFour Capital Corp.
By:	/s/ David A. Lorber
	Name:	David A. Lorber
	Title:	Authorized Signatory

/s/ David A. Lorber

DAVID A. LORBER

/s/ Stephen E. Loukas

STEPHEN E. LOUKAS

/s/ Zachary R. George

ZACHARY R. GEORGE

11

CUSIP NO. 670872100

SCHEDULE A

Transactions in Securities of the Issuer During the Past Sixty Days

Class of Security Amount of Securities Purchased/(Sold) Price ($) Date of Purchase/Sale

FRONTFOUR MASTER FUND, LTD.

Common Stock	3,448	30.6850	04/23/2015
Common Stock	553	30.8350	04/24/2015
Common Stock	14,398	29.2395	05/01/2015
Common Stock	983	27.3287	05/21/2015
Common Stock	1,419	27.5125	05/22/2015
Common Stock	3,000	26.5757	05/29/2015
June 2015 Call Option ($35 Strike Price)	(2)	0.4000	04/22/2015
June 2015 Call Option ($35 Strike Price)	(75)	0.3500	04/24/2015
June 2015 Call Option ($35 Strike Price)	(98)	0.4000	04/24/2015
June 2015 Call Option ($35 Strike Price)	(162)	0.2815	04/27/2015
June 2015 Call Option ($35 Strike Price)	(121)	0.3000	04/27/2015
June 2015 Call Option ($35 Strike Price)	(53)	0.3000	04/27/2015
June 2015 Call Option ($35 Strike Price)	(132)	0.2500	04/28/2015
June 2015 Call Option ($35 Strike Price)	(121)	0.2500	04/29/2015

FRONTFOUR OPPORTUNITY FUND

Common Stock	314	30.6850	04/23/2015
Common Stock	3,366	30.8350	04/24/2015
Common Stock	602	29.2395	05/01/2015
June 2015 Call Option ($35 Strike Price)	(4)	0.3500	04/24/2015
June 2015 Call Option ($35 Strike Price)	(9)	0.4000	04/24/2015
June 2015 Call Option ($35 Strike Price)	(15)	0.2815	04/27/2015
June 2015 Call Option ($35 Strike Price)	(11)	0.3000	04/27/2015
June 2015 Call Option ($35 Strike Price)	(5)	0.3000	04/27/2015
June 2015 Call Option ($35 Strike Price)	(11)	0.2500	04/28/2015
June 2015 Call Option ($35 Strike Price)	(12)	0.2500	04/29/2015

CUSIP NO. 670872100

FRONTFOUR CAPITAL GROUP LLC

(Through Separately Managed Accounts)

Common Stock	(2,500)	31.7421	04/06/2015
Common Stock	(404)	31.9322	04/13/2015
Common Stock	(700)	31.9229	04/16/2015
Common Stock	1,238	30.6850	04/23/2015
Common Stock	(5,000)	30.6564	04/23/2015
Common Stock	1,081	30.8350	04/24/2015
Common Stock	(5,000)	30.7826	04/24/2015
Common Stock	5,000	28.1293	05/08/2015
Common Stock	7,500	27.6761	05/12/2015
Common Stock	(7,500)	27.6016	05/12/2015
Common Stock	517	27.3287	05/21/2015
Common Stock	179	27.5125	05/22/2015
June 2015 Call Option ($35 Strike Price)	(30)	0.4000	04/22/2015
June 2015 Call Option ($35 Strike Price)	(21)	0.3500	04/24/2015
June 2015 Call Option ($35 Strike Price)	(43)	0.4000	04/24/2015
June 2015 Call Option ($35 Strike Price)	(91)	0.2815	04/27/2015
June 2015 Call Option ($35 Strike Price)	(68)	0.3000	04/27/2015
June 2015 Call Option ($35 Strike Price)	(30)	0.3000	04/27/2015
June 2015 Call Option ($35 Strike Price)	(75)	0.2500	04/28/2015
June 2015 Call Option ($35 Strike Price)	(67)	0.2500	04/29/2015