A sharp, wisecracking type in the early 1930s, O'Brien found his star persona becoming increasingly sentimentalized after the Production Code crackdown of 1934, but occasionally returned memorably to his earlier type, as in "Torrid Zone" (1940), opposite Cagney and Ann Sheridan. He remained a popular star through the 40s, often in stalwart roles such as not only his Rockne but also "The Iron Major" (1943). A childhood friend of Spencer Tracy, O'Brien acted with Tracy in "The People Against O'Hara" (1951) and in John Ford's sentimental roundup of veteran character players, "The Last Hurrah" (1958). Billy Wilder also put his iconic value to good use as an Irish cop on the tail of both the gangsters and the cross-dressing heroes on the run in the director's hilarious "Some Like It Hot" (1959).