[citation needed] The red-headed O'Brien used to make 10-hour return car trip to Glenelg to play under-15 games.

At 16 he played in his first West Gambier senior premiership in the Western Border league.

He has a terrific mark and good leg speed, where he recorded the third-best result in agility draft camp.

[2] On August 22, 2017, after having a career-best season, O'Brien signed a two-year contract extension keeping him at Hawthorn until the end of 2019.

[3] At the end of the 2021 AFL season, O'Brien exercised his rights as a free agent and joined the Western Bulldogs.