On 22 October 2006, O'Mahony won a Cork Premier Intermediate Championship medal after being introduced as a substitute in a 0-20 to 1-11 defeat of Carrigtwohill in the final.
He was a late replacement in goal on the Cork minor hurling team that won the Munster Championship following a 1-09 to 0-09 defeat of Clare in the final.
After becoming a dual player in 1991, O'Mahony won a Munster medal as a non-playing substitute with the Cork minor football team in 1991 after a two-point defeat of Kerry in the final.
O'Mahony was still eligible for the minor grade when he replaced Alan Hickey as first-choice goalkeeper on the Cork under-21 hurling team in 1992.
O'Mahony made his senior debut for Cork on 14 November 1993, replacing Paul O'Callaghan at half-time in a National League game against Galway at Páirc Uí Chaoimh.