 He successfully completed his Leaving Certificate and, not being the chosen for the seminary of Killaloe, O'Brien was directed instead to the Archdiocese of Dublin, where he began his studies in Clonliffe College in September 1949.He was ordained a priest by Archbishop John Charles McQuaid on 25 May 1956.O'Brien's first appointment was as chaplain to St. Michael's Hospital, Dun Laoghaire and afterwards, in 1957, he was made curate of Glasnevin Parish.