He was born in Dunedin, New Zealand in 1902 to John O'Shea, a law clerk, and his wife, Alice Marion Clark.

[1] He attended Otago Boys' High School where he excelled and initially intended to pursue a legal career.

O'Shea immediately began a strong recruitment campaign, making regular personal visits to all the provinces, encouraging them to join and strengthen their finances.

He always believed in free-trade ideals, arguing that protectionist measures and import licensing worked against New Zealand's agricultural trade.

He received the New Zealand 1990 Commemoration Medal, supposedly as the last surviving member of the Legislative Council (he was outlived by Ethel Gould by two years, though).