 O'Brien made his senior debut for Peel Thunder in the West Australian Football League in 2005 as a 16-year-old alongside his older brother, Rory O'Brien.He represented Western Australia in the national under 16 championships in 2004 and the national under 18 championships in both 2005 and 2006.He is also a graduate of the AIS/AFL academy and was a member of the national under 17 team which played against Ireland when they toured in 2006.