'Oleg Cassini (born Oleg Cassini Loiewski, 1913, Paris, France) is an American fashion designer.

He was raised in Italy, and moved to the United States in 1936, where he designed clothes for various Hollywood studios. He became a second lieutenant in the Army in World War II, at Fort Reilly, Kansas, and later returned to New York, where he designed ready-to-wear dresses while continuing to design for television and Broadway.

Cassini's first wife was cough-syrup heiress Merry Fahrney. They were married from September 2, 1938 until February 5, 1940. Cassini then married actress Gene Tierney on July 11, 1941. As Tierney's parents didn't approve of the union, the couple eloped to Las Vegas, and used earrings as wedding rings. The couple had two daughters, Antoinette Daria Cassini (born October 15, 1943) and Christine "Tina" Cassini (born November 19, 1948). Tierney contracted German measles during her first pregnancy, and their daughter, Daria, was born deaf, blind and mentally retarded. Tierney had an affair with John F. Kennedy, and the couple were divorced on February 28, 1952. At one time, Cassini was engaged to Grace Kelly.

In the 1960's, Cassini was chosen by Jacqueline Kennedy to design her state wardrobe. In 1987, he published an autobiography, In My Own Fashion (ISBN: 0671699067). He currently has a line of wedding dresses for David's Bridal.