He was born on 24 January 1926 at Ring, County Waterford, Ireland, a Gaelic-speaking regions in which John spoke Gaelic right from childhood years. His father was a farmer and lived his life close to the soil; there were five offspring in the family, three boys and two girls.
In 1939 John went as a boarder to St Augustine's College, Dungarvan, for his secondary education. He then joined the Augustinians. He made his Simple Religious Profession on 30 September 1945 at Orlagh in County Dublin, and was then sent to Rome to study at the Gregorian University. He was ordained to the priesthood in Dublin on 10 March 1951.
His first assignment was to the Augustinian parish at Hammersmith in London (1951 to 1954), then five years at Galway, and from 1959 to 1962 he assisted in the formation of Augustinian candidates living at Ballyboden and Orlagh, County Dublin. In 1963 he returned to Galway, and thence to Ballyhaunis in 1969-1971.
In 1972 he began twenty-seven years of ministry in Australia: Coorparoo (Brisbane) 1982 - 1986 (and parish priest here 1982-1986), Redland Bay (outer Brisbane) 1987 - 1989, Innisfail (north Queensland) 1991-1993, Manly Vale (Sydney) 1993-1994, Coorparoo again 1995-1998, and Kyabram (northern Victoria) until June 2000. In total, he ministered at Coorparoo for a far greater time than at any other place in Australia. Soon after Fr O’Sullivan returned to Ireland in mid-2000, the following tribute to him was published, “Father John O'Sullivan has a way with people, probably all people, and his gentle approach and happy smile have put him into a very special class of person.'' Even when in later years he lost his ability to move around freely, his mind was as active as ever, his great interest in people unaltered and his gentle humour undiminished.
Fr O’Sullivan died in Dublin on 25 April 2017, and his funeral Mass took place on 28 April at the Augustinian church in Cork, followed by his burial in the Augustinian section of the local cemetery.The first Augustinian came from Ireland to minister in Australia in 1938, and since then Augustinian presence in Australia has been continuous.