At the Battle of Mount Austen, the Galloping Horse, and the Sea Horse in January 1943, Oka and the 1st and 3rd Battalions from the 124th unsuccessfully attempted to defend a terrain feature called the Sea Horse from American attacks. After losing possession of the feature and surrounding areas, Oka and his surviving troops escaped to friendly lines. Japanese records indicate that he was killed soon after this, but some sources suggest he may have been evacuated with the rest of the Japanese forces during Operation Ke and survived the campaign. He was posthumously promoted to the rank of major general.