For his performance in The Siege of Jadotville (2016), he won an Irish Film & Television Award for Best Supporting Actor.

He had lead roles in other series, including The Agency, Band of Brothers, Monarch of the Glen, High Stakes, Playing the Field, The Bill, Berkeley Square, and Reach for the Moon.

In both 2005 and 2008, O'Mara guest-starred on TNT's The Closer as Bill Croelick, a charming and psychotic man with a fixation on fire.

[6] In October 2018, O'Mara joined the cast of The Man in the High Castle, where he portrays series regular Wyatt Price, a black market supplier of information to Juliana Crain about the comings and goings of alternate realities and their films.

[7] O'Mara married American actress Paige Turco in a Roman Catholic ceremony in Old Saybrook, Connecticut, in September 2003.