Educated at the Royal Naval College Dartmouth, O'Brien was commissioned into the Royal Navy in 1930. He served in World War II during which he served with the naval escort of the fateful PQ-17 convoy. In the early 1960s he served as Director of Naval Plans at the Ministry of Defence under Lord Louis Mountbatten.
He was then appointed Naval Secretary in 1964. He was appointed Flag Officer Aircraft Carriers in 1966, Commander-in-Chief, Far East Fleet in 1967 and Commander-in-Chief of the Western Fleet in 1970. He retired in 1971. In retirement he became Chairman of the King George's Fund for Sailors.
In 1975 he was Chairman of the Royal Navy Club of 1765 & 1785 (United 1889). He became Chairman of the Kennet and Avon Canal and after its re-opening by HM Queen Elizabeth in 1990, he retired from this post. He also held the posts of Rear-Admiral and then Vice-Admiral of the United Kingdom.
He died after a brief illness on 19 February 2016.