O’Brien grew up in Mount Gambier, on the South Australia and Victoria border. As a 16 year old he played in West Gambier’s Premiership in the Western Border League, repeating the success the following year.

Regularly making the 10 hour road trip to the city, O’Brien would represent Glenelg in SANFL junior competitions from an early age. At the draft camp O’Brien demonstrated his agility, finishing third in the testing despite being 193cm tall.

O’Brien was drafted by the Hawthorn Football Club with pick 28 in the 2012 National Draft. While his first season was spent developing in the VFL side, O’Brien broke into the senior side to make his debut in Round 1, 2014.

While used around the ground, O’Brien has made his home as a key defender. After 8 years at Hawthorn, O’Brien made the move to Footscray ahead of the 2022 season.

O’Brien played his 100th game early in 2022, fitting straight in to another strong premiership contending Dogs side.