Peter O'Toole belongs to a very interesting cohort of actors: classically trained Brits from working class backgrounds. They bring all of the techniques of the theater and Britain's literary tradition thanks to their training and education, but also a raw grit that you didn't see as much of in their more upper-class counterparts. You get the best of both a Brando and an Olivier with their work.
Along with Peter O'Toole and Richard Harris, you had Richard Burton, Robert Shaw, and (slightly younger) Nicol Williamson and the recently deceased Albert Finney. These "angry young men" of the stage and screen remain some of my favorites.