In 1873, O'Sullivan began his legal career in Ipswich, being employed by, and later articled to, Charles Frederick Chubb, Solicitor.

[2] In 1915, O'Sullivan was appointed as a District Court judge, and made chairman of the Central Sugar Cane Prices Board.

[2] O'Sullivan contested the electorate of Stanley, the seat his father had represented, at the 1899 election,[3] losing to the Ministerialist candidate, Frederick Lord.

[7] Within two weeks of losing his seat, O'Sullivan was reappointed to the Legislative Council,[1] the Premier this time being William Kidston.

He immediately returned to the role of Secretary for Agriculture in 1908,[1] and then in October of that year he was made Attorney-General and remained in the position until June, 1915.