In 1993, O'Shea was appointed Minister of State at the Department of Agriculture and Food with responsibility for Food and Horticulture. In 1994, he was appointed Minister of State at the Department of Health with responsibility for Mental Handicap, Public Health and Food Safety. He was party spokesperson for Defence (1997–1998); Education; Arts, Heritage, Gaeltacht and the Islands; Communications and Sport (1998–2002); Community, Rural and Gaeltacht Affairs (2002–2007); and Defence and the Irish language (2007–2011).