 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.He combined this with the first official diocesan chaplaincy to the Bon Secours Hospital.1962 brought a move to the rural setting of Ballitore in the parish of Narraghmore in County Kildare.