Having been with Rovers since the 2009-10 season, the midfielder has progressed each season with the club. Last term he was one of the star performers for the Academy as they reached the finals of both the FA Youth Cup and Academy League.
A regular on the international stage for the Republic of Ireland, O'Sullivan was recently called up to their Under-21 squad.
Made his debut for the first team in the final game of the 2012-13 season as a second half substitute against Birmingham City.