He began with a series of about eight books about Michael the O'Kelly. He then wrote three books that were more realistic in nature than the O'Kelly books and received a certain amount of critical praise. These books are: Crambo, Mills, and No Earth for Foxes.
There are a number of common characters in each of these books, such as Pavane and Crambo, but often the most important is Mills, who is obsessed, as apparently O'Brine himself was, with tracking down and killing Nazi war criminals. He also wrote the screenplays for films including Man from Tangier (1957) and The Long Shadow (1961) and episodes of television series including No Hiding Place and The Saint. His novel Passport to Treason was itself filmed in 1956 by producer-director Robert S. Baker, starring Rod Cameron.
R. Gordon Kelly notes that according to the backcover blurb for the 1976 American paperback edition of No Earth for Foxes, O'Brine was a former British secret agent who killed his first Nazi in Heidelberg in 1937 and his last one in Madagascar in 1950.