He plays as a back row forward, preferably at number eight. He is a native of Tralee. He formally played for Munster in the Magners League.
Before being recruited by his native province, O'Sullivan played for two seasons with Connacht. In the 2003–04 season he was voted Connacht's senior player of the year. He left Munster at the end of the 2008–09 season to join Agen.