For her role as "Beth" in Little Women (1949), she worked again with Mary Astor, who played "Marmee", her mother. Astor was also her mother in Meet Me in St. Louis (1944). In this film, she also worked again with Harry Davenport, who played "Dr. Barnes" and who played her grandfather in Meet Me in St. Louis (1944).
Gave birth to her only child, daughter Mara Tolene Thorsen. 
Her special Academy Award as Outstanding Juvenile Performer for Meet Me in St. Louis (1944) was stolen and she was unable to regain it for nearly fifty years when two memorabilia collectors came across it at a swap meet and managed to give it back to O'Brien.
In 1959, Ms. O'Brien starred in a national stage tour of "The Young And The Beautiful" by author Sally Benson (creator of the book that became O'Brien's most famous film, Meet Me in St. Louis (1944)). Co-starring opposite O'Brien in the stage play of "The Young And The Beautiful" was Dirk Wayne Summers, who later became an award winning writer and director in films and television.
In a practice common among child actors at the time, O'Brien adopted as her professional first name the name of the character who was her first credited part in Journey for Margaret (1942).
In April 2006 she was presented with one of the first two Lifetime Achievement Awards ever awarded by the SunDeis Film Festival at Brandeis University. Celeste Holm received the other.
In Italy, almost all of her films were dubbed by Loredana Randisi.
Measurements: 33-21-34 (at age 18), (Source: Celebrity Sleuth magazine)
Received the Women's International Center (WIC) Living Legacy Award. 
Stated on Turner Classic Movie's program "Private Screenings" (1995), in the "Child Stars" episode, that she is half Spanish.