O'Sullivan was born at Ipswich, Queensland, to Patrick O'Sullivan, himself a future member of the Queensland Parliament, and his wife, Mary. He was educated at the local state school and at St. Mary's College, Ipswich. In 1873, O'Sullivan began his legal career in Ipswich, being employed by, and later articled to, Charles Frederick Chubb, Solicitor. Two years later he moved to Brisbane, being articled to Robert Little, but returned to Ipswich to look after his brother's legal business after his death in 1877. He was admitted as a solicitorin September, 1878.
Legal career
Moving back to Brisbane in 1886, O'Sullivan went into partnership with Charles Bedell Lilley, son of Sir Charles Lilley, and after the partnership finished O'Sullivan continued on with the business while also studying for the Bar and was admitted in 1900. In 1911, he was made a KC. In 1915, O'Sullivan was appointed as a District Court judge, and made chairman of the Central Sugar Cane Prices Board. In 1922, he was made a judge in the Supreme Court of Queensland, serving until his retirement in 1926. During this time he was a member of the Court of Industrial Arbitration and chairman of the Land Appeal Court.
Political career
O'Sullivan contested the electorate of Stanley, the seat his father had represented, at the 1899 election, losing to the Ministerialist candidate, Frederick Lord. Within a week of having become Premier in 1903, Arthur Morgan appointed O'Sullivan to the Queensland Legislative Council. During his term he was the Government Representative in the Legislative Council. He resigned from the Council in January, 1906 to contest the seat of Warwick in a by-election to replace Arthur Morgan, who had resigned to join the Legislative Council. This time he was successful, winning by 449 votes to 435 against his competitor, George Barnes. He was appointed Secretary for Public Works in January 1906, serving until February 1907 when he was also given role of Agriculture, serving until November 1907. In the 1908 election, O'Sullivan was once again up against Barnes, this time the latter was successful, winning by 1297 votes to 1206. Within two weeks of losing his seat, O'Sullivan was reappointed to the Legislative Council, the Premier this time being William Kidston. He immediately returned to the role of Secretary for Agriculture in 1908, and then in October of that year he was made Attorney-General and remained in the position until June, 1915. He resigned in December 1915 to take up his appointment to the District Courts.
