He made his senior debut as a sixteen-year-old in 2002 as Ballygunner started a remarkable losing streak of being defeated in three successive club championship finals.
They broke this unenviable record at the fourth attempt with O'Sullivan winning his first championship medal following a 2-10 to 1-12 defeat of De La Salle.
[1] Ballygunner lost two further finals in succession before O'Sullivan won his second championship medal in 2009 following a one-point defeat of Lismore after a draw and a replay.
O'Sullivan later won his first Munster medal as Waterford defeated Cork by 3-16 to 1-21 to win one of the greatest games of hurling ever played.
After returning from a serious injury that kept him out of the game for six months, O'Sullivan added a National Hurling League medal to his collection in 2007 when Waterford defeated Kilkenny by 0-20 to 0-18 in the final.