[1] 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.
[1] After winning Peel's best and fairest award for a second time in 2010,[2] he transferred to East Fremantle for the 2011 season.
[2] During his time at East Fremantle, O'Brien has been awarded the Lynn Medal for the club's best and fairest on three occasions; in 2011 and 2014 he was the sole winner, while he shared the award with Andrew Stephen in 2013.
[3] In 2013, O'Brien won the Sandover Medal for the best and fairest player in the WAFL with 55 votes.
[4] After five years and 105 games with East Fremantle, O'Brien returned to his original club Peel Thunder for the 2016 season.