Although never a major star, Danish-born actress Osa Massen made an impact in such 1940s melodramas as A Woman's Face (1941), in which she engages in an outright catfight with heroine Joan Crawford, and the noir thriller Deadline at Dawn (1946), as a woman with something to hide. Trained as a newspaper photographer, Massen (born Aase Madsen) was persuaded by Danish director Alice O'Fredericks to make her acting debut in Kidnapped (1935), a comedy starring Denmark's answer to Shirley Temple, and although Osa had designs on a career as a film cutter, she agreed to appear in a second Danish film, the seemingly lost Bag Københavns Kulisser (1935). A screen test for 20th Century Fox led to a Hollywood contract. Director Edward H. Griffith cast her as a Dutch-Polynesian femme fatale in Honeymoon in Bali (1939), which several reviewers thought she stole outright from nominal stars Madeleine Carroll and Fred MacMurray.
Switching to Warner Bros., Massen appeared mainly in potboilers, her best assignment coming on loan to MGM in the aforementioned A Woman's Face, a remake of a Swedish melodrama that had starred Ingrid Bergman, with whom Massen was often compared. Playing leading roles in low-budget productions and supporting parts in Grade-A films, Osa, as many critics pointed out, always made her moments count. She scored as a mystery woman murdered on a train in Background to Danger (1943), a rather fanciful espionage thriller starring George Raft. Deadline at Dawn (1946), in which she played Paul Lukas' daughter, was one of the first true film noirs and Massen was again singled out by several critics. After being continually confused with Ona Munson and Hungarian import Ilona Massey, co-star Gene Raymond persuaded her to change her name to Stefanie Paull for Million Dollar Weekend (1948). She was back to Osa Massen in Rocketship X-M (1950), an early sci-fi thriller and perhaps her best-remembered film.
Divorced from Alan Hersholt, the son of character actor Jean Hersholt, Massen was widowed by her second husband, a Beverly Hills physician, in 1953. At that point, she concentrated on television guest roles. After appearing in shows ranging from Perry Mason to Wagon Train, Massen made her final screen appearance in Outcasts of the City (1958), a love story set in Germany and one of the last films released by Republic Pictures. Divorced from her third husband, a Hollywood dentist, she faded completely from public view.