Worked in banking before turning professional at the age of 50, O'Sullivan starred as an amateur golfer claiming the 1985 Irish Amateur Closed Championship and the Irish Amateur Stroke Play Championship in 1990.

Served for a short time as a Irish selector before winning his card at Senior Tour Qualifying School.

A six-time winner on the Senior Tour, O'Sullivan has finished inside the top ten on 44 occasions.

Became the first player to reach the milestone of 300 Senior Tour events at the 2016 Wales Senior Open.