Oleg Cassini was a fashion designer born to an aristocratic Russian family with maternal Italian ancestry. He came to the United States as a young man after starting as a designer in Rome, and quickly got work with Paramount Pictures. Cassini established his reputation by designing for films.

He was born in Paris as Oleg Aleksandrovich Loiewski, the elder son of Countess Marguerite Cassini and her husband Count Alexander Loiewski, a Russian diplomat, thereby obtaining the title of Count. His maternal grandfather Arthur Paul Nicholas Cassini, Marquis de Capuzzuchi di Bologna, Count Cassini, had been the Russian ambassador to the United States during the administrations of William McKinley and Theodore Roosevelt. Count Arthur Cassini signed the Treaty ending the war between Japan and China. His mother's family claimed Italian astronomer Giovanni Domenico Cassini as an ancestor. In 1918, the Russian Revolution caused the Loiewski family to flee for their lives, leaving behind their wealth, lands, and homes. As a young child, Oleg saw his cousin shot to death. The family reached Denmark and next moved to Switzerland. The Greek Royal family invited them to Greece, but, while traveling through Italy, revolution began in Greece as well. They got off the train in Florence, eventually settling there. In Italy the children started using the surname of Cassini.

On September 2, 1938, in Elkton, Maryland, Cassini became the husband of Mary "Merry" Fahrney (1910-1974). The couple divorced in 1940. Cassini married actress Gene Tierney, on June 2, 1941. Their daughter, Antoinette Daria, was born on 15 October 1943 blind, deaf and with severe brain damage. She died on 11 September 2010. The couple separated on 20 October 1946, and entered into a property settlement agreement on 10 November 1946 in which Cassini's fifty percent share of the marital estate was dedicated to the ongoing care of Daria. Tierney won an uncontested divorce in California on 13 March 1947, and the divorce would have been finalized one year later, on 13 March 1948. However, according to Cassini's diary, they reconciled some months before that. They announced that they were back together in April 1948. Gossip columnist Hedda Hopper reported the news in the Los Angeles Times on 9 April 1948. Their second daughter, Christina "Tina" was born on 19 November 1948 and died in 2015. They finally divorced in 1953.