O'Brien made his senior debut on 21 April 2001 in a 1-16 to 0-13 group stage defeat of Laois in the National League. Tipperary later claimed the league title, however, O'Brien remained as a substitute for that victory. Later that year he made his championship debut when he came on as substitute for Lar Corbett in the provincial decider against Limerick. Tipperary triumphed by 2-16 to 1-17, with O'Brien collecting his first Munster medal. He later collected an All-Ireland medal as a non-playing substitute as Tipperary defeated Galway by 2-18 to 2-15.