南西方面艦隊
On 25 August 1942, Oishi was assigned command of "Aikoku Maru", a merchant raider, and was promoted to captain the same year. In September, "Aikoku Maru" was assigned to the Southwest Area Fleet (IJN 8th Fleet) and tasked to transport elements of General Tanayoshi Sano's IJA 38th Division to Rabaul for the reinforcement of Guadalcanal. In November 1942, while on a raiding mission in the Indian Ocean, "Aikoku Maru" attacked the Royal Dutch Shell oil tanker "Ondina". In December 1943-April 1944, "Aikoku Maru" participated in reinforcement operations to New Guinea.