She was a favorite of both James Whale and John Ford. For Whale, she made memorable supporting appearances in The Invisible Man (1933) and The Bride of Frankenstein (1935). She played more serious characters for Ford in 1935’s The Informer (as Wallace Ford's mother) and The Plough and the Stars (1936).
O’Connor seems to have made a movie with almost every big Hollywood star at one time or other. She was in three Errol Flynn films: The Adventures of Robin Hood (a delightful turn as Marian’s maid-in-waiting), The Sea Hawk, and The Adventures of Don Juan. She also appeared in films with Ronald Colman (Random Harvest), Barbara Stanwyck (Christmas in Connecticut), Bob Hope (My Favorite Spy), Bing Crosby (The Bells of St. Mary’s), Norma Shearer and Fredric March (The Barretts of Wimpole Street), and Jennifer Jones (Cluny Brown).
Her most frequent co-star was Charles Laughton. They worked together in The Barretts of Wimpole Street, This Land is Mine, The Canterville Ghost, Forever and a Day, and Witness for the Prosecution. Indeed, Witness for the Prosecution provided O’Connor with her most famous role, as the hard-of-hearing housekeeper Janet McKenzie. She played the part for two years on Broadway and reprised it for Billy Wilder’s film version. It was her final performance.
Una O’Connor died of heart disease in 1959. She never married.