Oona O'Neill, Lady Chaplin (14 May 1925 – 27 September 1991) was an English-American actress who was the daughter of Irish-American playwright Eugene O'Neill and English-born writer Agnes Boulton, and the fourth and last wife of English actor and film-maker Charlie Chaplin.

O'Neill's parents divorced when she was four years old, after which she was raised by her mother in Point Pleasant, New Jersey, and rarely saw her father. She first came to the public eye during her time at the Brearley School in New York City between 1940 and 1942, when she was photographed attending fashionable nightclubs with her friends Carol Marcus and Gloria Vanderbilt. In 1942, she received a large amount of media attention after she was chosen as "The Number One Debutante" of the 1942–1943 season at the Stork Club. Soon after, she decided to pursue a career in acting and, after small roles in two stage productions, headed for Hollywood.

In Hollywood, O'Neill was introduced to Chaplin, who considered her for a film role. The film was never made, but O'Neill and Chaplin began a romantic relationship and married in June 1943, a month after she had turned 18. The 36-year age gap between them caused a scandal, and severed O'Neill's relationship with her father, who was only six months older than Chaplin and who had already strongly disapproved of her wish to become an actress. Following the marriage, O'Neill gave up her career plans. She and Chaplin had eight children together and remained married until his death. The first decade of their marriage was spent living in Beverly Hills, but after Chaplin's re-entry permit to the United States was cancelled during a voyage to London in 1952, they moved to Manoir de Ban in the Swiss village of Corsier-sur-Vevey. In 1954, O'Neill renounced her US citizenship and became a British citizen. Following Chaplin's death in 1977, she split her time between Switzerland and New York. She died of pancreatic cancer at the age of 66 in Corsier-sur-Vevey in 1991. Her daughter Geraldine Chaplin named her daughter after Oona in 1986.