Adelaide's first pick in the 2014 rookie draft, O'Brien spent his early years behind Sam Jacobs before taking the No.1 ruck role in 2019 when Jacobs hurt his knee. The 201cm ruckman won the Malcolm Blight Medal in 2020 and, in 2024, collected his third Phil Walsh Best Team Man Award along with the AFL's Jim Stynes Community Leadership Award. He has played 148 games and kicked 17 goals.