Trivia

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.

Gave birth to her only child at age 39, a daughter Mara Tolene Thorsen, on July 12, 1976. Child's father is her second husband, Roy T. Thorsen.

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.

Stated on Turner Classic Movie's Child Stars (2006), that she is half Spanish.

For her role as "Beth" in Little Women (1949), she worked again with Mary Astor, who played her mother "Marmee", and also played 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), and also with June Allyson, who had already played her sister in Music for Millions (1944).

Received the Women's International Center (WIC) Living Legacy Award. (1996)

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.

Was suggested for the role of Emmeline as a child in The Blue Lagoon (1949) but an agreement between MGM, the company where she was under contract, and Rank, the company that financed the project, could not be reached.

In Italy, almost all of her films were dubbed by Loredana Randisi.

Began wearing a nose ring in 2010.

In 1949 Margaret was announced for the voice of Alice in the upcoming Walt Disney animated production of Alice in Wonderland (1951), but details could not be worked out, and the voice of Alice was provided by Kathryn Beaumont.