Born in Bishopstown, Cork, O'Neill first played competitive hurling whilst at school in Coláiste an Spioraid Naoimh. He arrived on the inter-county scene at the age of seventeen when he first linked up with the Cork minor team, before later joining the under-21 side. He joined the senior team as a member of the extended panel during the 2005 championship. Since then O'Neill has become a regular member of the starting fifteen and has won one Munster medal on the field of play. He has been an All-Ireland runner-up on one occasion.