 In June 1940, he was assigned command of the IJA 124th Infantry Regiment, which participated in combat operations in the Second Sino-Japanese War.He was then scouted by General Kiyotake Kawaguchi for the proposed invasion of Port Moresby in New Guinea and reassigned to the southwest Pacific front in 1942, landing on Guadalcanal together with the forces commanded by Colonel Kiyonao Ichiki.Oka commanded a portion of Japanese troops from the 35th Infantry Brigade in a losing effort during the Battle of Edson's Ridge in September, 1942.