[4] While studying for his master's degree from 1871 to 1874, O'Connor worked as a clerk in the New South Wales Legislative Council, after which he was articled with Frederick Darley (afterwards Chief Justice), remaining solvent with contributions to the Freeman's Journal, the Echo and the Evening News.

[4] In the dispute between protection and free trade that formed the backbone of Australian colonial politics in the late nineteenth century, O'Connor was a committed protectionist.

[5] The protectionist George Dibbs appointed him Minister for Justice on 23 October 1891, a position in which he worked closely with his friend Barton, who was Attorney General.

[3] O'Connor remained in the Legislative Council but embarked on an overseas tour, travelling to Egypt, Italy, England and Ireland.

[3] In 1895, O'Connor returned to New South Wales and successfully defended the protectionist demagogue Paddy Crick against conspiracy charges relating to the George Dean attempted murder case; he was appointed Queen's Counsel in 1896 and acted as a judge on the Supreme Court from November 1898 to March 1899.

He organised the "nexus clause" which guaranteed the Senate half as many members as the House of Representatives, arguing that this would act as an incentive to keep parliamentary numbers, and therefore expenditure, comparatively low.

[4] O'Connor resigned from the Legislative Council on 16 July 1898,[5] in order to contest the lower house seat of Young against the sitting Labour member, Chris Watson.

The number of salaried ministers was capped by the Constitution, but O'Connor was forced to write to the Attorney General, his friend Alfred Deakin, that he could not continue to work without remuneration.

[3] On 29 July 1903, O'Connor introduced legislation for the establishment of a High Court of Australia, describing it as vital in "maintaining the balance of the Constitution".

[3] O'Connor was a liberal justice who saw judges as "not only the interpreters, but also the guardians of the Constitution", advocating the Court's primacy over the Judicial Committee of the Privy Council on these matters.

[3] O'Connor, still a sitting justice, died from pernicious anaemia at St Vincent's Hospital in the suburb of Darlinghurst on 18 November 1912, aged 61; he had suffered from chronic nephritis for some years.

[4] After his death he received tributes from, among others, Barton, who believed "that assiduous toil did much to shorten a life that was most precious", and Griffith, who described O'Connor as "absolutely fearless in the performance of his judicial duties".

[3] Billy Hughes, then serving as Attorney General in the Fisher Labor government, recalled that O'Connor "never exhibited any trace of personal enmity".