O'Marra played in his first international tournament with Canada Ontario in the 2004 World U-17 Hockey Challenge, defeating Canada Pacific to earn the gold medal. He then won a gold medal as the alternate captain for Team Canada at the 2004 U-18 Junior World Cup. He also played on the Canadian under-18 team that finished with a silver medal at the 2005 IIHF World U18 Championships in Plzeň, Czech Republic.