Born Raymond in 1946 in Waterford, Ireland, he moved to Swindon, England, with his family at the age of eleven. There he began to play the piano, an activity he continued while studying at an art school. Looking for a manager, O'Sullivan meets
Gordon Mills, who built Tom Jones and Engelbert Humperdinck.

In addition to the idiosyncratic look, there was also a name change: Raymond was dropped in favor of Gilbert. In 1971 he writes his first big hit "Alone Again (Naturally)", which becomes number one in the USA, he succeeds in other bestsellers with "Get Down", "What's Another Kiss" and the beautiful ballad "Clair".