As filed with the Securities and Exchange Commission on June 16, 2015

Registration No. 333-            

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549

FORM S-1

REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

OM Asset Management plc
(Exact name of each registrant as specified in its charter)

Stephen H. Belgrad
Chief Financial Officer
c/o Old Mutual (US) Holdings Inc.
200 Clarendon Street, 53^rd Floor
Boston, Massachusetts 02116
(617) 369-7300
(Name, address, including zip code, and telephone number, including area code, of agent for service)

Approximate date of commencement of proposed sale to the public:
As soon as practicable after the effective date of this Registration Statement.

          If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.    o

          If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ý 333-204799

          If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

          If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

          Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of "large accelerated filer," "accelerated filer" and "smaller reporting company" in Rule 12b-2 of the Exchange Act.

CALCULATION OF REGISTRATION FEE

(1)

Includes shares subject to the underwriters' option to purchase additional shares.

(2)

In accordance with Rule 462(b) under the Securities Act of 1933, as amended, an additional amount of securities having a proposed maximum aggregate offering price of no more than 20% of the maximum aggregate offering price of the securities eligible to be sold under the related Registration Statement on Form S-1 (File No. 333-204799), as amended is hereby registered.

(3)

Calculated pursuant to Rule 457(a) under the Securities Act of 1933, as amended.

          This Registration Statement shall become effective upon filing with the Securities and Exchange Commission in accordance with Rule 462(b) promulgated under the Securities Act.

EXPLANATORY NOTE AND INCORPORATION OF
CERTAIN INFORMATION BY REFERENCE

        This Registration Statement on Form S-1 is being filed with the Securities and Exchange Commission (the "Commission") pursuant to Rule 462(b) under the Securities Act of 1933, as amended, by OM Asset Management plc (the "Registrant"). This Registration Statement incorporates by reference the contents of, including all exhibits to, the Registrant's Registration Statement on Form S-1 (File No. 333-204799), as amended, which was declared effective by the Commission on June 16, 2015 (the "Prior Registration Statement").

        The required opinion and consents are listed on an Exhibit Index attached hereto and filed herewith.

SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, Commonwealth of Massachusetts, on the 16th day of June, 2015.

        Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

2

3

EXHIBIT INDEX

4

QuickLinks