After his promotion to ensign in 1921, he was assigned to the destroyer Yukaze, followed by the oiler Shiretoko, and then the battleship Mutsu.

He subsequently served in a number of staff positions, including assignments to the 1st Carrier Strike Force of the Combined Fleet and 1st Air Fleet, participating in all major carrier operations from the attack on Pearl Harbor to the Battle of Midway.

On 25 August 1942, Oishi was assigned command of Aikoku Maru, a merchant raider, and was promoted to captain the same year.

In November 1942, while on a raiding mission in the Indian Ocean, Aikoku Maru attacked the Royal Dutch Shell oil tanker Ondina.

On 3 April 1943, Oishi was recalled to Japan, and held several shore positions until the end of the war.