O'Sullivan first came to prominence on the inter-county scene as a dual player at minor level in the early 1980s. He won both Munster and All-Ireland medals with the hurlers in 1979. Two years later in 1981 O’Sullivan won both Munster and All-Ireland medals with the footballers. The following year he won another brace of Munster and All-Ireland honours with the under-21 hurlers. In 1984 he won Munster and All-Ireland medals with Cork’s under-21 football team.
By this stage O’Sullivan had joined the Cork senior hurling team. He won his first Munster title in 1982, however, Cork were later defeated by Kilkenny in the All-Ireland final. In spite of this loss his performance earned him his first All-Star award. O’Sullivan came on as a substitute in 1983 to win a second Munster medal, however, once again Kilkenny accounted for the Leesiders in the All-Ireland final. The following year O’Sullivan won a second Munster title and, once again, Cork qualified for the championship decider. In a special All-Ireland final played at Semple Stadium in Thurles Cork defeated Offaly, giving O’Sullivan his first senior All-Ireland medal. He won a fourth provincial medal in 1985, however, Cork were later defeated by Galway in the All-Ireland semi-final. 1986 saw O’Sullivan win a fifth consecutive Munster medal, a win which later allowed Cork to qualify for the All-Ireland final. In spite of being ranked as undergogs, Cork defeated Galway and O’Sullivan won a second All-Ireland medal. His performances during the year later earned him a second All-Star.
Cork lost their provincial crown for the next few seasons, however, O’Sullivan was Cork’s sole All-Star winner in 1988. Two years later in 1990 he added a sixth Munster medal to his collection. Once again Cork faced Galway in the subsequent All-Ireland final and, once again, Cork were the rank outsiders. In a thrilling and high-scoring game Cork accounted for the men from the west and O’Sullivan picked up a third All-Ireland medal. Once again he collected an All-Star award and was also named as the Texaco Hurler of the Year. Two years later O’Sullivan won a seventh provincial title, before lining out in a sixth All-Ireland final. Unfortunately, victory went to Kilkenny on the day, however, O’Sullivan was honoured with a fifth All-Star award. In 1993 he won a National Hurling League medal after a three-game saga with Wexford. This was O’Sullivan’s last major honour as a player and he retired from inter-county hurling shortly afterwards.