Osato began her career at the age of fourteen with Wassily de Basil's Ballets Russe de Monte-Carlo, which at the time was the world's most well known ballet company; she was the youngest member of the troupe, their first American dancer and their first dancer of Japanese descent. De Basil tried to persuade Osato to change her name to a Russian name, but she refused to do so. She spent six years touring the United States, Europe, Australia and South America with the company, leaving in 1941 as she felt her career was stagnating. She went to study at the School of American Ballet in New York City for six months, then joined the American Ballet Theatre as a dancer. While at the ABT, she danced roles in such ballets as Kenneth MacMillan's "Sleeping Beauty", Antony Tudor's "Pillar of Fire", and Bronislava Nijinska's "The Beloved".