On 7 March 2009, he lined out at left corner-back when UCC defeated the University of Limerick by 2-17 to 0-14 to win the Fitzgibbon Cup.
[2] O'Sullivan joined the Sarsfields club at a young age and played in all grades at juvenile and underage levels.
On 27 October 2007, he partnered Daniel Kearney at midfield when Sarsfields defeated Midleton by 2-15 to 1-08 to win the Premier Minor Hurling Championship title.
[3] On 3 May 2008, O'Sullivan made his first appearance for the Sarsfields senior team when he lined out at left corner-back in a 2-09 to 2-08 defeat of Ballinhassig.
[7] Sarsfields again failed to retain the title in 2012 after suffering a two-point defeat by Midleton, however, O'Sullivan played in his sixth final in seven seasons on 12 October 2014.
O'Sullivan made his first appearance on 23 November 2008 when he lined out at right corner-back in a challenge game to celebrate the 150th anniversary of St. Colman's College.
[17] On 3 July 2014, an ongoing hamstring injury meant O'Sullivan was named on the bench when Cork faced Limerick in a second successive Munster final.
Team selector Johnny Crowley said: "“Obviously, we’d have liked if Conor had stayed...it was his choice, we asked him to think about it but he said he wanted to focus on his club hurling.