[1] O'Sullivan played in all grades of hurling with Midleton CBS Secondary School before progressing onto the college's senior team.
On 12 March 2006, he scored a point from centre-back when Midelton CBS defeated St Flannan's College from Ennis by 2-08 to 0-12 to win the Harty Cup.
After capturing three successive divisional junior football championship medals between 2008 and 2010, O'Sullivan's side reached the county decider at the third attempt.
[6] On 25 June 2006, O'Sullivan scored a point from left wing-back when Cork defeated Tipperary by 2-20 to 1-15 to win a second successive Munster Championship title.
[10] O'Sullivan returned to the Cork senior panel during the 2008 National League before making his first Munster Championship appearance on 8 June 2008 in a 1-19 to 1-13 defeat by Tipperary.
[11] Two months before Cork's opening championship game in 2013, O'Sullivan suffered a serious leg break in a club match which ruled him out for the rest of the season.