But after his father enlisted during the Second World War, his mother could no longer run the farm and raise the children by herself, so the family returned to Katanning and stayed there for seven years.

The boom in wool prices at the time of the Korean War inflated his family's income, allowing them to send O'Halloran to Guildford Grammar School in Perth,[2] the state's capital city.

[2] O'Halloran attracted the attention of the leading Western Australian coach, Don Gravenall, but his schoolwork limited him to a few weeks of intense training over Christmas.

In his final year at Guildford, O'Halloran was the School Captain, led the swimming and shooting team, and was a member of the rowing eights in the Head of the River.

O'Halloran made his national debut at the 1955 Australian Championships in Adelaide; he finished fifth in the 110 yd freestyle behind future Olympians Jon Henricks and John Devitt.

[3] Upon the recommendation of his parents, O'Halloran moved to Sydney in late 1955 to train with Frank Guthrie in an attempt to qualify for the 1956 Summer Olympics in Melbourne, Australia.

"[3] O'Halloran refined his style and increased his workload to around 10 km a day, something that was normal for competitive swimmers in the eastern states, but uncommon in Western Australia.

He did this despite suffering from ear trouble, making him the fifth-fastest swimmer in the world for the 1956 calendar year, which earned him an individual berth in the 400 m event at the Olympics.

[6] Devitt, Chapman, Graham Hamilton and Garretty finished third in their heat behind Great Britain and the Soviet Union, and qualified fifth-fastest behind Japan and the United States.

[8] With four of the five fastest individual swimmers in the calendar year for the event,[9] Australia was heavily favoured to win the relay; Sports Illustrated predicted a world record and a victory margin of around 14 metres.

[2] O'Halloran's selection also broke up the team of Devitt, Rose, Chapman and Henricks, which had won the relay for New South Wales at the Australian Championships.

[4] O'Halloran led off and put Australia in the lead with an Olympic record time of 2 min 6.8 s,[1] opening a gap of 0.7 s over the Soviet Union and 1.2 s over the United States.

Australia never relinquished the lead and steadily increased it, as Devitt, Rose, and Henricks set the three fastest splits in the race, faster than all the non-Australian swimmers.

[12] Swimming from lane seven, O'Halloran cut almost 4 s off his personal best but lost the bronze medal to the United States' George Breen by 0.4 s, in a race won by Rose.

Despite his achievements, the state government ignored calls for Western Australia to build an Olympic standard swimming pool until 1962, when they constructed one for the 1962 British Empire and Commonwealth Games in Perth.

[6] In 1958, O'Halloran's parents travelled across the continent to watch him swim at the Australian Championships in Sydney, but a recurring ear infection hindered his performances.