O'Brien made his Australian representative debut in the coxed four at the World Junior Rowing Championships in 2016. They rowed to a fifth placing. In 2018 he was selected with
Andrew Judge in a coxless pair to compete at both the
World Rowing U23 Championships and the
World Championships. They finished in seventh pace at the U/23 championships and in 13th place at the World Championships in Plovdiv. In 2019 O'Brien was selected in the Australian senior men's sweep squad for the international representative season. 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. Rowing with
Tim Masters,
Nicholas Purnell and
Jack Hargreaves, O'Brien took the gold medal in the Australian coxless four at both the World Rowing Cup II in Poznan and at WRC III in Rotterdam. The four were looking for a top eight finish at the 2019 World Championships to qualify for the
Tokyo Olympics. They won their heat and semi-final, thereby qualifying the boat for
Tokyo 2020. In Tokyo the Australian men's eight placed fourth in their heat, fourth in the repechage and sixth in the Olympic A final. 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. With O'Brien at stroke they took gold at the
World Rowing Cup II in Poznan in June, won the
Stewards Challenge Cup at the
Henley Royal Regatta in July and a week later finished second at the WRC III in Lucerne. At the
2022 World Rowing Championships at Racize, he stroked the Australian coxless four to a silver medal. At the
Rowing World Cup II in Varese, Italy O'Brien raced as Australia's 2- entrant with
Angus Dawson. They finished 3rd in the A final to win the bronze medal. At 2023's RWC III in Lucerne, O'Brien and Dawson were selected into the Australian men's eight. In the final the Australian eight rowed stroke for stroke with their fancied Great Britain rivals but then moved away at the 1000m mark and held on for an upset gold medal victory. 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. They won their heat powering past the USA eight who had headed them at the 1000m mark. In the A final Australia and Great Britain traded the lead over the first 1000m, but beyond that point the result mirrored that of 2022 with Great Britain exerting dominance by the 1500m, fighting off a fast finishing Dutch eight who took silver and leaving the Australians with the bronze for the second successive year. ==References==