Born in Southampton, O'Sullivan first came to prominence as a Gaelic footballer after moving to Cork. He enjoyed success at juvenile and underage levels with the Bishopstown club, however, it was as a member of the club's under-21 hurling team that he won Cork Under-21 Championship medals in 2006 and 2007. O'Sullivan subsequently won a Sigerson Cup medal with University College Cork in 2011.[2]
O'Sullivan never played for Cork at minor level; he was added to the under-21 panel in 2009 and ended the year with an All-Ireland Championship medal.[3] He joined the Cork senior football panel the following year and played a key role in helping the team to their first All-Ireland Championship title in 20 years. O'Sullivan also won the first of three successive National Football League medals that year before completing his silverware collection in 2012 by winning a Munster Championship medal.[citation needed]
O'Sullivan announced his retirement from inter-county football on 22 December 2018.[4]