Enlisting in the Citizen Military Forces and later the Australian Imperial Force, during World War II he saw service in New Guinea, serving with the signal corps of the 3rd Division with the rank of captain.

[1] At the 1959 state election, O'Neil contested the seat of Canning, located in the southern suburbs of Perth, despite having only joined the Liberal Party the previous year.

Court initially governed in coalition with the National Country Party, led by Ray McPharlin, but the coalition collapsed in May 1975, and O'Neil consequently replaced McPharlin as deputy premier when a new ministry was constituted the following month, also becoming Minister for the North-West.

The ministry was again reconstituted following the 1977 election, with O'Neil losing the housing, works, and water portfolios, but gaining the roles of Chief Secretary, Minister for Police, and Minister for Regional Administration.

[1] O'Neil remained deputy premier and minister until his retirement at the 1980 state election, with Anthony Trethowan succeeding him in East Melville.