Born 14 May 1878 at Christchurch, New Zealand, the seventh child and fifth son of Michael O'Brien, who owned a boot-factory there, and his wife Bridget Anne Quinn. He was educated at Christchurch Boys High School, and come to England to take his medical training at Guy's Hospital, where he served as house physician, after winning the gold medal and taking honours in surgery, midwifery, and diseases of women at the London University qualifying examination in 1905. He held resident posts at the East London Children's Hospital, Shadwell, and at Moorfields Royal London Ophthalmic Hospital. He proceeded to the MD in 1906 and took the Fellowship in 1912. He then returned to New Zealand, and was elected FRACS in 1929. O'Brien married in 1904 Mary Catherine Carter, who survived him with four sons and two daughters. He practised for many years at 130 Hereford Street, Christchurch, and lived at 70 Papanui Road, where he died on 31 May 1951, aged 73.