O'Brien's next big showcase came with "Meet Me in St. Louis" (1944). As Tootie Smith, the feisty but fragile little sister of Judy Garland, she was a bright point in a very good film, especially in her musical numbers with Garland and during a Halloween sequence in which she confronts a grouchy neighbor. For her performance, she was awarded a special juvenile Oscar. Her next two features, "Music for Millions" (1944) and the drama "Our Vines Have Tender Grapes" (1945) were also impressive, but her luck pretty much wore out after that. Her last MGM films were generally unimpressive: the Western "Bad Bascombe" and the comedy "Three Wise Fools" (both 1946) and the melodrama "The Big City" (1948). Two good roles came her way in 1949, as the tragic Beth in an otherwise unremarkable remake of "Little Women" and as Mary Lennox in "The Secret Garden."
O'Brien left MGM after that and her film career pretty much tapered off. She played her first love scene (at age 14) in the appropriately-titled low-budget "Her First Romance" (1951) for Columbia and had ingenue roles in "Glory" (1955) and in the all-star Western "Heller in Pink Tights" (1960). Her only other films to date have been the Disney-produced period drama "Amy" (1981) and a cameo in the direct-to-video horror spoof "Sunset After Dark" (1994).
But as soon as her film contract had ended, the teenaged actress plunged into "the Golden Age of Television". Deluged with offers, O'Brien acted on such anthology series as "Studio One", "The Lux Video Theater", "Ford Television Theater", "Playhouse 90" and "The June Allyson Show". O'Brien reprised her big screen role of Beth in a TV musical version of "Little Women" (CBS, 1958), alongside Florence Henderson, Jeanie Carson and Joel Grey. A pilot for her own series, the domestic sitcom "Maggie" (CBS, 1960), did not fly. But as she aged from teen to slightly plump young lady and into svelte, lovely middle age, O'Brien continued to appear on the small screen from time to time, turning up in such longforms as the "Ironside" TV-movie "Split Second to an Epitaph" (NBC, 1968) and the miniseries "Testimony of Two Men" (syndicated, 1977) and making guest appearances on such series as "Love, American Style" (1968), "Adam-12" (1971), "Marcus Welby, M.D." (1972) and "Murder, She Wrote" (1991). O'Brien has also appeared onstage in summer stock and cruise ship productions of "Barefoot in the Park", "Under the Yum-Yum Tree", "A Thousand Clowns" and others.