O'Sullivan was born in Paddington, New South Wales and was the son of a publican. He was educated at the Christian Brother's School, Paddington and initially worked as an apprentice coach painter with the New South Wales Government Railways. He became active in the Federated Coachmakers' Employees Union but took over the licence of his father's hotel, The Lord Dudley, and then continued in a career as a publican. He was active in community organizations in the Paddington area and was elected as an alderman on Paddington Municipal Council between 1923 and 1934. He was the mayor in 1927

O'Sullivan was elected to parliament at the 1927 election for the newly re-established seat of Woollahra. Due to a redistribution the seat became untenable for Labor at the 1930 election and he transferred to the seat of Paddington. The sitting Nationalist member for Paddington, Daniel Levy, successfully contested Woolahra. O'Sullivan retained Paddington at the next 9 elections and retired from parliament at the 1959 election . He was a member of the Australian Labor Party (NSW) while that party was separated from the Federal executive of the Labor Party between 1931 and 1936 and was the party whip from 1932 till 1941.^[1]

With the election of the Labor government of William McKell at the 1941 election, O'Sullivan was appointed Minister for Transport, a position he retained until 1950. He was then commissioned as the Minister for Health and he stayed in this post until 1956. He retired in 1959.^[1]