In 2007, he was a member of the Ireland Under-20 Grand Slam winning squad. O'Brien was called up to the Ireland team for the 2009 November Tests, and he gained his first cap when he came on as a substitute against Fiji at the RDS Arena on 21 November 2009. He also came on as a replacement against South Africa in a 15–10 win on 28 November 2009. He came off the bench against Italy in the 2010 Six Nations Championship, but was dropped afterwards. He was injured during Ireland's 2010 Summer Tests, but won his first start against Samoa in the 2010 Autumn Tests, though he did not feature in any other games. O'Brien started every game for Ireland during the 2011 Six Nations, winning the Man of the Match award against Italy.