Born 28 July 1906 at Oamaru, New Zealand, son of John O'Brien, of Devonport, NZ, civil servant, and Minnie Cagney, his wife. He was educated by the Marist Brothers at Vermont Street, Dunedin, and at the Sacred Heart College there. He graduated in medicine in 1930 from the Otago Medical School, and came to England for postgraduate study at St Bartholomew's Hospital, and also worked on the continent and in Ireland. He took the Fellowship, though not previously a Member of the College, on 10 December 1936. On returning to New Zealand O'Brien served as deputy superintendent of the Kawakawa Hospital and then as medical superintendent of the Bay of Islands Hospital. He married in January 1939, settled as a consultant at Auckland, and was elected assistant surgeon to the Mater Misericordiae Hospital. He acted as secretary of the Auckland division of the British Medical Association. At the outbreak of war in September 1939 he joined the NZ Medical Corps, and went on service in the Pacific in January 1942. Two years later he was transferred to the Middle East, and later served at a base hospital in Italy and then, as a major, commanded a field surgical unit in the Italian front line. He was invalided to Egypt at Christmas 1944, and while at No 5 NZ General Hospital at Helwan was shot and killed in the spring of 1945 by a drunken sentry, who was convicted of his murder. A requiem mass was held at the Roman Catholic Cathedral at Auckland, at which the bishop spoke of his "largeness of mind and gaiety of heart". O'Brien married at St Patrick's Cathedral, Auckland, in January 1939 Katherine Smyth, who came from Ireland to marry him. Mrs O'Brien survived him, with a son and a daughter.