- The first time she met Clark Gable he was in old-man make-up for Strange Interlude (1932). He invited her to go horseback riding, but she turned him down. Later when she met him a second time to record voice-overs, she realized his true age and regretted her decision. He never asked her out a second time.
- She used to make Irish soda bread for Greta Garbo.
- Along with Joyzelle Joyner, she was one of only two credited cast members of Just Imagine (1930) who were still alive in 1980, the year in which the film takes place.
- Despised working with the chimpanzee Cheetah during the filming of the Tarzan movies at MGM and, according to daughter Mia Farrow, privately referred to the primate as "that ape son of a bitch".
- Her oldest son, Michael Farrow, was killed in a plane crash while taking flying lessons (1958).
- Was a favorite of Irving Thalberg and Louis B. Mayer at MGM and they had big plans for her as a big star. Thalberg's sudden death at age 37 of pneumonia in 1936 put a big damper on the momentum in her pursuit of stardom and was soon relegated to romantic interest roles.
- Mother of Michael Farrow, Patrick Farrow, John Charles Farrow, Mia Farrow, Tisa Farrow, Prudence Farrow and Stephanie Farrow.
- Irish-born O'Sullivan was sent by her father, a British army major, to Roehampton, a convent school just outside of London, because her brogue had become so thick. She was two years older than Vivien Leigh, her best friend at the school. While Leigh was determined to be an actress, O'Sullivan's ambition was to be an aviatrix.
- She played the mother of her real life daughter Mia Farrow in Hannah and Her Sisters (1986).
- In May 1934, she received police protection after reports surfaced that she was in danger of being kidnapped.
- She met future husband John Farrow, who was a writer on the Fox lot, when she was there to make Just Imagine (1930).
- Following her death, she was interred at Most Holy Redeemer Cemetery in Niskayuna, Schenectady County, New York.
- Is often dubbed 'Ireland's first film star'.
- Was friends with: Maureen O'Hara, Vivien Leigh, Betty Furness, Robert Ryan, Johnny Weissmuller, and Mr. and Mrs. Lyndon B. Johnson.
- She was of Irish, with some English and Scottish, ancestry.
- Of her later films, she liked The Tall T (1957) the best.
- Was discovered in Dublin by director Frank Borzage, who was in Ireland shooting exteriors for Song o' My Heart (1930).
- Paramount Pictures insisted she do a screen test for her husband John Farrow's picture, The Big Clock (1948).
- Is represented with an Audio Animatronic figure in The Great Movie Ride in the Tarzan scene, at Disney's Hollywood Studios at Walt Disney World. After the ride's closure in 2017, the figure was dressed in different clothes and moved-into the Pirates of the Caribbean ride.
- Became a stage actress at Pat O'Brien's urging for a Chicago production of "A Roomful of Roses" (1961).
- She appeared in three films that have been selected for the National Film Registry by the Library of Congress as being "culturally, historically or aesthetically" significant: Tarzan and His Mate (1934), The Thin Man (1934) and The Tall T (1957).
- She was a very active member of both the Hollywood Democratic Committee and The Hollywood Anti-Nazi League and donated her time and money to many liberal causes (such as the creation of the United Nations and the Civil Rights Movement) and political candidates (including Franklin D. Roosevelt, Henry Wallace, Adlai Stevenson, John F. Kennedy, Robert F. Kennedy, Jimmy Carter, and Bill Clinton) during her lifetime.
- She and John Farrow were married at St. Monica's Church in Santa Monica, California. Among the groomsmen were Alan Mowbray and Ainsworth Morgan.
- She was the last surviving cast member of Just Imagine (1930).
- Was a supporter of: UNICEF, The United Nations, The Democratic National Committee, and the Habitat for Humanity.
- She appeared in stage revivals of "Pygmalion" and "The Glass Menagerie".
- She appeared in films with both Will Rogers and Will Rogers Jr..
- Grandmother of Fletcher Previn, Ronan Farrow, Moses Farrow, Soon-Yi Previn, Daisy Previn and Dylan O'Sullivan Farrow.
- O'Sullivan and Robert Young were paired in five films at MGM: Strange Interlude (1932), Tugboat Annie (1933), West Point of the Air (1935), The Emperor's Candlesticks (1937) and Sporting Blood (1940).
- Honored by a day of her films being shown during TCM's "Summer Under the Stars" event on 8 August 2022.
- She was awarded a Star on the Hollywood Walk of Fame at 6541 Hollywood Boulevard in Hollywood, California on February 27, 1991.
- She appeared alongside Lew Ayres in three films at MGM: Okay America! (1932), Spring Madness (1938) and Maisie Was a Lady (1941).
- Interviewed in Tom Weaver's book "I Was a Monster Movie Maker" (McFarland & Co., 2001).
- She has appeared in two films that have been selected for the National Film Registry by the Library of Congress as being "culturally, historically or aesthetically significant": Tarzan and His Mate (1934) and The Thin Man (1934).
- Before her death, Woody Allen and Maureen had not spoken in years. He learned of her passing from the news and was angry that Mia Farrow did not inform him directly.
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