In 1938, he ran on the American Labor ticket for New York State Attorney General, but was defeated.

He was New York State Commissioner of Standards and Purchase when Governor Herbert H. Lehman appointed O'Leary New York State Comptroller after the death of Morris S. Tremaine to fill the vacancy.

Tremaine died on October 12, 1941, and O'Leary was appointed on October 17 with the understanding that he was nominated to run on the Democratic and American Labor tickets at a special election for Comptroller in November, but on October 27 the New York Court of Appeals ruled that no such special election is to be held and O'Leary continued in office until the end of 1942.

He was a founding member of the Liberal Party of New York, and was its Secretary in 1958.

He died on December 29, 1964, at Beth Israel Medical Center in Manhattan.