Talented leading man of the 1940s turned veteran Hollywood character player, at his best playing roles requiring sweat and strain. O'Brien joined Orson Welles' Mercury Players in 1937 and worked in film and on stage through the 1940s, turning primarily to the screen after WWII. He is best remembered as the impetuous young street poet in "The Hunchback of Notre Dame" (1939); the enterprising insurance investigator of "The Killers" (1946); the doomed lead in the noir classic, "D.O.A." (1949); the sycophantic Hollywood press agent in "The Barefoot Contessa" (1954), which won him a Best Supporting Actor Oscar; and for a string of weather-beaten character parts in the 70s. O'Brien was married to actresses Nancy Kelly and Olga San Juan; Maria O'Brien, his daughter by the latter, has appeared in several films.