Olga Kosakiewicz (Russian: Ольга Козакевич, born 6 November 1915 Kiev – 1983) was a student of Simone de Beauvoir who joined the circle of de Beauvoir and Jean-Paul Sartre in the autumn of 1935 when she was 19. She and her sister, Wanda Kosakiewicz, are fused together to make one central character in de Beauvoir's first novel L'Invitée (She Came to Stay, 1943, Gallimard), which was dedicated to Olga (where her name appears as Kosakievicz in the Norton translation).
In Sartre's trilogy of novels, Les Chemins de la Liberté (The Roads to Freedom), the character of Ivich is considered a representation of Olga.
Deirdre Bair's biography of Simone de Beauvoir examines this relationship. Hazel Rowley also discusses it at length in her book about the relationship between Simone de Beauvoir and Jean-Paul Sartre.
Olga married Jacques-Laurent Bost, a long-time lover of de Beauvoir. She died of tuberculosis in 1983.