Career
He is the older brother of Brock O'Brien. O'Brien made his league debut for Peel Thunder in round 1 of the 2005 season and was voted the club's best and fairest player in 2006. He spent the 2008 season at North Adelaide Football Club in the South Australian National Football League, before returning to Peel Thunder in 2009.
After winning Peel's best and fairest award for a second time in 2010, he transferred to East Fremantle for the 2011 season. 2011 also saw O'Brien make his state debut for Western Australia. After five years and 105 games with East Fremantle, O'Brien returned to his original club Peel Thunder for the 2016 season.