O'Shanassy was elected to the inaugural Victorian Legislative Council for City of Melbourne in 1851. When Victoria gained responsible government in 1856, he was elected to the Legislative Assembly for both Melbourne and Kilmore districts, he decided to represent the latter resulting in a by-election for Melbourne. In 1868 he returned to the Council as member for Central Province, then in 1877 he went back to the Assembly as member for Belfast, which he represented until 1883. Although he had been regarded as a liberal in his early career, in the colonial Parliament O'Shanassy emerged as the leader of the conservative opposition to the reforming ministry of William Haines, although there was no formal party system in Victoria at this time. His alienation from the liberals was mainly as a result of the tariff issue: most colonial liberals were protectionists, but O'Shanassy was a free trader. When Haines's ministry fell in March 1857, O'Shanassy succeeded him as Premier, but his government fell in April and Haines returned to office. O'Shanassy was Premier again from March 1858 to October 1859 and his deputy was Charles Gavan Duffy. The combination of these two Irish Catholics heading up the colony's administration was a source of great consternation among the Protestant elite and ensured that sectarian issues emerged in the election of 1859. However, the Duffy/O'Shanassy link strained, possibly, because O'Shanassy departed Ireland before the turmoil of 1848, while Duffy was involved in the 1848s troubles. O'Shanassy was Premier again from November 1861. By the 1860s O'Shanassy had become a thorough-going conservative and his government was bitterly opposed by the liberal newspaper The Age and its fiery proprietor, David Syme. In retaliation, O'Shanassy withdrew government advertising from the paper. O'Shanassy's government was finally defeated in June 1863 over the land reform issue, and he never held office again. O'Shanassy was awarded a papal knighthood in 1859, but news of that award did not reach Victoria until early 1860. The award was pilloried in the Melbourne Punch. Further, he received CMG in 1869 and a in 1874. He retired from Parliament in February 1883, shortly before his death in Boroondara, Victoria, Australia.
