He won five Commonwealth Games gold medals and claimed a total of nine individual and six relay titles at the Australian Championships, before retiring at the age of 21 due to financial pressures.
After showing promise at an early age, O'Brien was sent to Sydney to train under renowned coach Forbes Carlile and his breaststroke assistant Terry Gathercole.
O'Brien successfully defended both his breaststroke titles at the 1966 British Empire and Commonwealth Games in Kingston, Jamaica before retiring to support his family.
There were not many non-sporting activities for children in Wellington, and O'Brien played basketball and rugby league, did athletics and swimming, and rode horses.
[1] He added a second gold as part of the New South Wales team that won the 4 × 100 m medley relay in a time of 4 min 18.3 s. His performances gained him selection for the 1962 British Empire and Commonwealth Games in Perth.
[5] He then completed his campaign with a victory in the 4 × 110 yd medley relay, combining with Julian Carroll, Kevin Berry and David Dickson to complete the race in a time of 4 min 12.4 s.[1][3][5] In 1963, O'Brien captured the breaststroke double at the Australian Championships, setting personal bests in both events, and was a member of the New South Wales team that won the medley relay.
His performances earned him selection for an overseas tour to Europe with the Australian team, competing in the Soviet Union, Germany and England before visiting Japan and Hong Kong.
O'Brien joined the rest of the team for the national camp before the Olympics in Ayr in northern Queensland, where he trained under head coach Don Talbot.
The favourites for the 200 m breaststroke were Chet Jastremski of the United States—the world record holder—and Georgy Prokopenko of the Soviet Union.
[4] O'Brien was known for the strength that his torso generated, and his powerful kicks; sports science experiments showed that his vertical jump was especially strong.
He often gained a lead of approximately a metre from his dive and underwater glide at the start, and was able to complete 50 m in 31.0 s.[6] During the final training sessions in Tokyo, Talbot organised time trials for the Australians, which were held in front of opposition swimmers in an attempt to intimidate them.
O'Brien's acceleration in the third meant that he tired at the end, but he had enough energy to fend off Prokopenko in the late stages to win the gold medal in a new world record time of 2 min 27.8 s, a margin of 0.4 s, with Jastremski a further 1.4 s in arrears.
[3][4] The Australian coaches rested O'Brien for the heats of the 4 × 100 m medley relay; Peter Tonkin swam the breaststroke leg instead.
Undergoing a crash diet and fitness program, O'Brien lost 12.7 kg in twelve weeks of intense training.
O'Brien was unable to reclaim either of his individual Australian titles, but New South Wales again won the medley relay.
O'Brien did better in the newly introduced 100 m event, winning his heat in a time of 1 min 8.9 s to qualify second-fastest for the semifinals.
Along with Michael Wenden, Robert Cusack and Karl Byrom, the Australian quartet won their heat and entered the final as the equal fifth fastest qualifier.
[12] O'Brien also rued the absence of Talbot to motivate him to work, and had a further accident at the Olympic Village when his fingers were slammed by a closing window.
Television and camera work had always interested O'Brien, and he secured a job as a stagehand for Channel Nine after returning from the Tokyo Olympics, which he held for more than ten years.