O'Shanassy was born near Thurles in County Tipperary, Ireland, the son of a surveyor, and came to the Port Phillip District (later Victoria) in 1839.

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.

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.(G.

Serle) 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.

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.