He was a Labor Party Member of the New South Wales Legislative Assembly spanning 27 years, representing Eastern Suburbs between 1920 and 1927 and Orange between 1941 and 1947.

[1] In the 1927 split that divided the Labor Party, O'Halloran (Caucus Secretary at the time), sided with the McKell faction against Premier Jack Lang.

O'Halloran subsequently lost endorsement for this move and was not pre-selected for the newly formed seats of Bondi, Coogee, Randwick, Vaucluse, Waverley and Woollahra.

In 1944, it was reported that O'Halloran, a resident of Chandos Street in Ashfield, was fined £10 in the Central Police Court of Sydney for having driven a motor car while under the influence of liquor.

The Magistrate, in fining O'Halloran, said he was not prepared to extend the benefit of hardship, meaning that the licence is automatically cancelled for twelve months.