Olivér Halassy won Olympic silver in 1928 under his birthname of Haltmayer, winning gold in 1932 and 1936 as Halassy. In addition, he was a three-time European Champion in water polo (1931, 1934, 1938). As the age of eight years he had his left lower leg amputated after a traffic accident with a tram, so he was one of the first physically disabled athletes at the Olympic Summer Games. Nevertheless, he was 1931 European Champion in the 1500 metre freestyle and won a total of 25 individual Hungarian national titles in swimming between 1926 and 1938 over 400, 800, and 1500 metre freestyle, in the 4×200 freestyle relay and open-water swimming, adding nine with the team in open-water swimming. With his club he also won 10 consecutive Hungarian Championships in water polo (1930-39). Halassy was capped 91 times for Hungary. As a swimmer he set 12 national records, and in 1978 he was inducted into the International Swimming Hall of Fame.
After his swimming career Halassy went on to become an auditor in City Hall. In September 1946, at the age of just 37, after a taxi ride home in Budapest, he was murdered by a Russian soldier during a night mugging.