O'Sullivan made his intercounty debut for Dublin in 2009. He won the All-Ireland Senior Football Championship with Dublin in September 2011 against Kerry at Croke Park. He won the National Football League with Dublin 2013 playing in midfield at Croke Park against Tyrone. The game finished on a scoreline of 0-18 to 0-17. On 22 September 2013, he started in midfield for the All Ireland Final. He was a key performer in the game ultimately moving to corner back following an injury to Jonny Cooper in Dublin's defeat of Mayoon a scoreline of 2-12 to 1-14. He won his first all star at centre back following displays of versatility throughout the year. O'Sullivan was crucial to Dublin's success in 2015. Following defeat to Donegal in 2014, manager Jim Gavin deployed him as a "sweeping centre back", responsible for shoring up Dublin's defense against leaking goals. He became the lynchpin of Dublin's team with his reading of the game, calmness in possession and athleticism key to Dublin conceding only 4 goals in their 7 championship games played, of which Dublin kept 4 clean sheets. Despite suffering a hamstring injury in the final minutes of the semi final replay defeat of Mayo, O'Sullivan started the final where Dublin defeated reigning champions Kerry 0-12 to 0-09, playing just over an hour of the game. He received his second All Star award at centre back. He was again a key performer in 2016 as Dublin retained both their National League and Leinster titles. He also played a key role as the Dubs defeated Mayo by a single point after a replay on a scoreline of 1-15 to 1-14 to retain the Sam Maguire Cup. In 2017 Dublin were narrowly defeated in the National League final by Kerry by a single point. They then went on to win a record 7 Leinster titles in a row. On 17 September, O'Sullivan was heavily involved as Dublin claimed a historic 3 in a row All Ireland titles with another narrow 1-17 to 1-16 victory against Mayo. Having been outplayed in the first half, the Dubs turned the game around to win a thrilling game courtesy of a 75th minute Dean Rock free. O'Sullivan won his third all star following his displays.
