O'Brien was drafted 29th overall by the Ottawa Senators in the 2007 NHL Entry Draft. O'Brien played defense before switching in high-school to "play as a power-forward center-ice man" His final Central Scouting rank was 38th. He was picked by Ottawa general manager Bryan Murray because "size matters". O'Brien is a strong skater, he blocks shots and he is a penalty killer. It was the third year in a row that the Senators picked a US-born player. They picked Nick Foligno 28th overall during the 2006 NHL Draft, and Brian Lee ninth overall in 2005.