He studied at the Popular Song School of Giichi Tsukamoto.
Haruo Oka started as a street singer and worked as a shop assistant before he became a disc artist of the King Gramophone Record Company. He debuted with song "Kokkyo no Haru" ("Spring at the Border") in 1939. In 1944, during the Pacific War, he was dispatched to Ambon Island, but soon returned due to sickness. After the war, his popularity grew, and he starred in Akogare no Hawaii koro with Hibari Misora. However, he never performed at the Kohaku Uta Gassen partly because he attached importance to live performances.
Haruo Oka married Kiyoko Okuda in 1940.