 He was educated at Eton College, the Royal Military College, Sandhurst, early in the First World War, and Magdalen College, Oxford.He married, on 13 December 1921 Anne Molyneux Thesiger (1898-1973), daughter of Frederic Thesiger, 1st Viscount Chelmsford.O'Brien was commissioned into the British Army's Rifle Brigade in 1916 and served with their 3rd Battalion in the First World War.