O'Brien's looks belied the fact that she primarily performed in comedic roles during the height of her formal film career. This was in part due to her intentionally humorous singing style, which involved her singing in a deadpan manner, with no facial expressions and very little movement - reportedly she stumbled upon this "gimmick" by accident during a stage show when she became virtually paralyzed with stage fright before singing a number. The audience found the performance to be hilarious and she was soon hired to repeat this performance in a number of movies beginning in 1940 for which she gained the nicknames "Frozen Face" and "Miss Ice Glacier" among others. It should be noted that when she wasn't singing, her acting style was just as emotive as other actresses, and she didn't always employ her gimmick when singing as evidenced by her performance in the excerpt from Show Boat in the 1946 film Till the Clouds Roll By.
Among the films she appeared in during her time at MGM were The Big Store (1941) with the Marx Brothers, Ship Ahoy (1942) with Eleanor Powell and Red Skelton, Thousands Cheer (in which she endured ribbing from Mickey Rooney about her singing style), Du Barry Was a Lady (with Skelton and Lucille Ball), The Harvey Girls (with Judy Garland) and Ziegfeld Follies. After appearing once again with Red Skelton in 1947's, Merton of the Movies, and after a guest appearance the following year in the short Musical Merry-Go-Round, O'Brien was suddenly dropped from her MGM film contract and she moved into television and back to live performances.
She made two film appearances after this: Francis in the Navy and the 1976 Walt Disney Studios comedy, Gus. In 1984 she created a cabaret act saluting her career with MGM and this was recorded at the Masquer's Club in Hollywood, subsequently released as a Compact Disc and then with iTunes.
She continued to perform well into the 1980s with both a one-woman show and a production of Show Boat. She also continued to perform in the 1990s with the Palm Spring Follies.
|Francis in the Navy||1955||Nurse Kittredge|
|Musical Merry-Go-Round||1948||(Herself) Specialty Singer|
|Merton of the Movies||1947||Phyllis Montague|
|Till the Clouds Roll By||1946||Ellie Mae in 'Show Boat'/Specialty Singer|
|Ziegfeld Follies||1946||Specialty Singer|
|The Harvey Girls||1946||Alma from Ohio|
|Two Girls and a Sailor||1944||(Herself) Specialty Singer|
|Meet the People||1944||"Woodpecker" Peg|
|Thousands Cheer||1943||(Herself) Specialty Singer|
|Du Barry Was a Lady||1943||Ginny|
|Panama Hattie||1942||Flo Foster|
|Ship Ahoy||1942||Fran Evans|
|Lady Be Good||1941||Lull|
|Ringside Maisie||1941||(Herself) Specialty Singer|
|The Big Store||1941||Kitty|
|Sky Murder||1940||Lucille LaVonne (uncredited)|