Una O'Connor (October 23, 1880 – February 4, 1959) was an Irish actress who worked extensively in theater before becoming a notable character actress in film.

Born Agnes Teresa McGlade in Belfast, Northern Ireland, O'Connor began her acting career with Dublin's Abbey Theatre. For many years she worked in Ireland and England as a stage actress, and appeared in Alfred Hitchcock's Murder! (1930). Despite her lengthy apprenticeship she had not attracted much attention until she was chosen by Noel Coward to appear in Cavalcade. Her success led her to Hollywood to reprise her role, and with its success, O'Connor decided to remain there.

A favourite of the director James Whale, among O'Connor's most successful and best remembered roles are her performances in Whale's The Invisible Man (1933) and Bride of Frankenstein (1935). O'Connor's other notable films include The Barrets of Wimpole Street (1934), The Personal History, Adventures, Experience, & Observation of David Copperfield the Younger (1935), The Informer (1935), Father Brown, Detective (1935), Little Lord Fauntleroy (1936), The Strawberry Blonde (1941), My Favourite Spy (1942), Random Harvest (1942), The Canterville Ghost (1944), Christmas in Connecticut (1945), The Bells of St. Mary's (1945), Cluny Brown (1946) and Adventures of Don Juan (1948).

O'Connor also appeared in supporting roles in various theater productions, and achieved an outstanding success in the role of "Janet McKenzie" in Agatha Christie's Witness for the Prosecution on Broadway from 1952 until 1954. As one of the witnesses, in what was essentially a serious drama, O'Connor's character was intended to provide comic relief, as so many of her past characterisations had. O'Connor was highly praised for her work, and also played the role in the Billy Wilder film version of the same name in 1957. The film was a great success, and O'Connor once again received excellent reviews. It was her final film performance. By this time she was in her late seventies, and decided to retire.

She died in New York, New York from heart disease.

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