[1][2] Born in Dubbo, New South Wales,[3] O'Brien was boarding at Tudor House School in Bowral from an early age and then attended St Joseph's College Hunters Hill where he took up rowing.

[7] At the New South Wales State Championships in February 2020 rowing with Jack Hargreaves, he won the men's elite pair.

In an effort to qualify the men's pair for the 2020 Olympics, selectors broke up the Australian dual world champion coxless four into other boats giving O'Brien an opportunity in the four.

[10] O'Brien, Hargreaves, Hill, and Purnell were selected to race Australia's coxless four at the 2019 World Rowing Championships in Linz, Austria.

[14] In 2022 O'Brien regained a seat in the reigning Olympic men's champion four with Hargreaves, Purnell and Turrin as the coxless four in the Australian squad for the 2022 international season and the 2022 World Rowing Championships.

[17] At the Rowing World Cup II in Varese, Italy O'Brien raced as Australia's 2- entrant with Angus Dawson.

[10] For the 2023 World Rowing Championships in Belgrade Serbia, the Australian men's eight was left unchanged and O'Brien again raced in the six seat.