Though best known to cult film fans for his roles in Italian B-movies, Donald O'Brien hailed from Pau, in the Pyrenees mountains of France. His Irish father was a former US Army Calvary officer who fought in the Spanish-American war, and his mother was an English governess. When World War II broke out and France came under Nazi occupation, the O'Briens fled back to Ireland, where Donald would spend his formative years. He studied acting under the Irish theatre legend Micheál MacLiammóir, and later moved back to France, where he worked several oddjobs including prizefighter and office worker. In 1953, the 23-year-old O'Brien made his first appearance in a feature film, Anatole Litvak's war drama Act of Love (1953). A chance meeting with director John Frankenheimer saw him cast as a Nazi officer in The Train (1964), which so impressed the director that he cast him in a supporting role in Grand Prix (1966). In 1967, O'Brien was brought to Italy to star in Sergio Sollima's cult Spaghetti Western Corri uomo corri (1968). His portrayal of ex-American lawman turned soldier of fortune Nathaniel Cassidy led to future leading roles in the genre for a number of years, during which he changed his name from "Donal" to "Donald" due to contracts frequently misspelling it. O'Brien quickly became a staple of Italian B-movies, appearing in everything from Spaghetti Westerns, to horror films, to Sexploitation pictures. In 1981, he starred in Zombie Holocaust (1980) (retitled 'Doctor Butcher, M.D.' for its US release), which earned him a strong cult following among horror and exploitation film fans for his portrayal of the eponymous mad scientist. However, that same year he sustained a brain injury that paralyzed half his body, and significantly reduced the number of roles he could play even after he recovered. He appeared in several films for the notorious Joe D'Amato, and had a supporting role as a Franciscan friar opposite Sean Connery in Jean-Jacques Annaud's The Name of the Rose (1986). Following another accident in 1996, O'Brien all but retired from acting, settling in Paris with his family.