[3] O'Sullivan first stood as a candidate for Dáil Éireann at the by-election on 7 November 1979 in the Cork City constituency, following the death of Labour TD Patrick Kerrigan.
The by-election was won by Fine Gael's Liam Burke, but O'Sullivan was successful at his next attempt, when he topped the poll at the 1981 general election in the new constituency of Cork North-Central.
Along with the other Labour ministers, he resigned on 20 January 1987 in protest at proposed cuts in health spending, bringing about the coalition's collapse.
Six years later, after the 1992 general election led to a coalition with Fianna Fáil, O'Sullivan became Chairman of the Dáil's Enterprise and Economic Strategy Committee.
After the collapse of that government and its replacement with the Fine Gael–Labour Party–Democratic Left Rainbow Coalition, Taoiseach John Bruton appointed him in December 1994 as Minister of State at the Department of Tourism and Trade, a position he held until Fianna Fáil returned to power at the 1997 general election.