O'Sullivan was signed by MGM in 1932, where she would remain for the next decade. She soon played--for the first of six times--the most famous role of her career, the demure but scantily clad Jane, saved by and later wed to the king of the jungle in "Tarzan the Ape Man" (1932). The other five films O'Sullivan made with former Olympic swimmer Johnny Weismuller ranged from the superior "Tarzan and His Mate" (1934) to the odd "Tarzan's New York Adventure" (1942) before O'Sullivan retired for six years to raise her family by director John Farrow, whom she had married in 1936.
Remarkably prolific in the 30s, O'Sullivan wasn't quite a top star, typically playing leads in "B" films and programmers, and ingenue second leads in bigger productions. Examples of the former include the punchy gangster saga "Okay America" (1932), the spicy pre-Production Code dramas "Skyscraper Souls" (1932) and "Stage Mother" (1933), the superior horror film "The Devil Doll" (1936), the amusing comedy "The Bishop Misbehaves" and the fine suspenser "Woman Wanted" (both 1935). Always in gentle, sympathetic roles, O'Sullivan handsomely supported major stars like Norma Shearer in "The Barretts of Wimpole Street" (1934, very touching as the helpless Henrietta), Marie Dressler in "Tugboat Annie" (1933), Ann Harding in "The Flame Within" (1935), Greta Garbo in "Anna Karenina" (1935), and William Powell and Myrna Loy in "The Thin Man" (1934, as a damsel in distress). She provided the requisite romance in the Marx Brothers farce "A Day at the Races" (1937), dallied with Robert Taylor in "A Yank at Oxford" (1938), and was particularly fine as the spirited Greer Garson's sweet, more conventional sister in "Pride and Prejudice" (1940).
Returning to acting after six years, O'Sullivan played prominent but somewhat incidental romantic leads in two good noirs directed by Farrow, "The Big Clock" (1948) and "Where Danger Lives" (1950). Too old for ingenue roles and working in a Hollywood whose dominant Irish female was the more fiery Maureen O'Hara, O'Sullivan alternated standardized leading lady roles with supporting turns in good second-tier films like Douglas Sirk's "All I Desire" (1953) and Budd Boetticher's "The Tall T" (1957). Occasional clinkers like "Bonzo Goes to College" (1952) paled beside good TV work on the dramatic anthologies "Fireside Theater", "Matinee Theater" and "Star Stage". She also hosted a syndicated TV anthology for the young, "The Children's Hour" (1951).
After Farrow died in 1963 and her seven children (including actors Mia and Tisa Farrow) grew into adulthood, O'Sullivan increasing busied herself with stage work, which, over the next several decades included revivals of "The Front Page" and "Morning's at Seven". The Broadway hit "Never Too Late" had O'Sullivan sweetly amusing as a middle-aged expectant mother, a role she recreated for the 1965 film version. TV work picked up in the 70s and beyond with a season on the daytime soap "All My Children" and dignified roles in the TV-movies "The Crooked Hearts" (ABC, 1972), "Mandy's Grandmother" (syndicated, 1980) and "With Murder in Mind" (CBS, 1992). A handful of feature roles cropped up too: O'Sullivan proved anew that a sensitive actor existed alongside the persona of the likable, demure lassie with her striking work as the heroine's (Mia Farrow) drunken mother in Woody Allen's "Hannah and Her Sisters" (1986).