Born in Yanyarrie in outback South Australia, the Irish Catholic O'Halloran was educated at public schools before leaving to work on his parents' farm aged 13.

Despite this, or possibly because of this, O'Halloran comfortably retained Frome for the rest of his life, and succeeded Robert Richards as Leader of the Opposition on 17 October 1949.

[2] During his eleven years as ALP leader, O'Halloran lost four consecutive elections to the Sir Thomas Playford IV led Liberal and Country League.

Notwithstanding his own Catholicism, he resisted overtures to join the Catholic-dominated Democratic Labor Party, ensuring that the South Australian branch of the ALP remained free from the splits that occurred in Victoria and Queensland.

Sharp philosophical differences (Playford was a liberal conservative, O'Halloran was a trade-union socialist) did not prevent the two men from being friends.

Many of Playford's policies were socialistic by the standards of the time (such as the development of government-owned electricity boards) and as such were anathema to his conservative colleagues.

Since the LCL never held more than 23 seats during Playford's tenure, he often required Labor support to get many of these policies past his own party's right wing.

For most of the 1950s, the public believed the key to South Australia's ongoing economic success was the status quo of Playford as Premier and O'Halloran as opposition leader.

When he sadly mentioned this fact to Playford, the latter (who, as a Baptist, had no great interest in meeting the Pope) made arrangements to visit Pius XII while in Europe to attend Queen Elizabeth II's coronation.

Similarly, Don Dunstan recalled an incident during the 1953 election campaign when O'Halloran arrived at a public meeting intoxicated, and had to be led away quietly before he fell over.

Labor's most effective orator prior to Dunstan's entry into politics, O'Halloran was a "decent, heavily built but gentle man who ... wore an air of sleepy benevolence", smoked a pipe and spoke with an Irish brogue.