Born in Dublin, O'Sullivan was educated at a Christian Brothers school, then joined the Irish Defence Forces during The Emergency, serving in the Cavalry Corps.
In 1946, he left the Army to work for Bord na Móna, also joining the Local Government and Public Services Union (LGPSU).
[1] O'Sullivan gradually came to prominence in the LGPSU, serving on its executive committee and as president from 1962 before he was elected as general secretary in 1964,[2] in which role he was known for his calls for a national wages policy.
[1] In 1983, O'Sullivan stood down from his trade union posts to join the National Planning Board.
He later became acting director-general of the Institute for Industrial Research and Standards, and completed a master's degree and doctorate with Trinity College Dublin, with a focus on Irish history.