It was during her return stay in Tunisia that Ostermeyer began participating in sports, competing in basketball and track and field events. After the war, she continued her participation in athletics while resuming her education at the Conservatoire. She competed in a range of contests, eventually winning French titles at running, throwing and jumping events. In 1946, she placed second in the shot put at the European Athletics Championship in Oslo, as well as winning the Prix Premier at the Conservatoire.
The 1948 Summer Olympics were Ostermeyer's finest hour as an athlete. She won gold medals in the shot put and discus throw (despite having picked up a discus for the first time just a few weeks before the event), and a bronze medal in the high jump. She thus became the first Frenchwoman to win three medals in a single summer or winter Olympic Games. Her performance was only overshadowed by that of Fanny Blankers-Koen, who won four gold medals at the same Olympics. After winning the shot put, she concluded the day with an impromptu performance of a Beethoven concert at her team's headquarters.
She retired from sports in 1950 after having won two medals at that year's European Championships, and continued to pursue a career in music. Her athletic prowess damaged her reputation as a concert pianist, however, and she even avoided playing anything composed by Franz Liszt for six years because she considered him too "sportif". She toured for fifteen years before personal commitments, including the death of her husband, led her to take a teaching job, a post she held until her retirement in the early 1980s. In her final years she emerged from retirement to give a series of concerts in both France and Switzerland before her death in Bois-Guillaume.