On March 30, 2006, at the conclusion of the 2005–06 season, O'Marra was signed to a three-year contract by the New York Islanders. He then joined the Islanders' American Hockey League affiliate, the Bridgeport Sound Tigers, scoring two goals in his professional debut. O'Marra was then returned to the Otters for his final junior-eligible season, in 2006–07. The Otters got off to a slow start, and he was traded to the Saginaw Spirit in late November in exchange for promising centre Zack Torquato. On February 27, 2007, O'Marra's NHL rights were dealt in a trade to the Edmonton Oilers, along with Robert Nilsson and the Islanders' first-round draft pick in the 2007, in exchange for Ryan Smyth. After two difficult seasons with the Oilers' minor league affiliates, the Springfield Falcons and Stockton Thunder, O'Marra made his NHL debut with Edmonton in the 2009–10 season. He played six minutes and was a –1 plus-minus in a game against the Ottawa Senators on November 10, 2009. O'Marra later recorded his first career NHL point on an assist of a Colin McDonald goal against the Vancouver Canucks, on November 28; the goal was also McDonald's first career NHL point. O'Marra scored his first career NHL goal on December 26, 2010, against Cory Schneider of the Canucks. On March 17, 2011, due to multiple injuries to Oilers players, O'Marra was again recalled to the NHL, along with fellow Oklahoma City Barons – Edmonton's new AHL affiliate – teammates Alexandre Giroux and Chris VandeVelde. During the following season, 2011–12, O'Marra was traded to the Anaheim Ducks in exchange for Bryan Rodney on February 16, 2012. On July 27, 2012, with the impending 2012–13 NHL lockout, O'Marra signed his first European contract on a one-year deal with Lahti Pelicans of the FinnishSM-liiga. However, after only eight games into the 2012–13 season and enduring a diminished role within the roster, O'Marra was mutually released from his contract on October 4, 2012. O'Marra then played nine games for HC Fassa in the Italian Serie A in the regular season before moving to Vålerenga in Norway for the rest of the regular season and playoffs. With Vålerenga, he had seven goals and six assists for 13 points in 15 games in the playoffs. After playing one season in Italy with Val Pusteria HC, O'Marra signed with Elite Ice Hockey League team Coventry Blaze in the United Kingdom for the 2014–15 season. After playing one season with the Blaze, O'Marra announced his retirement from professional ice hockey on March 26, 2015. He continued to play hockey in Alberta at the senior men's level with the Stoney Creek Generals of the Allan Cup Hockey.