軍令部
After his promotion to captain on 1 December 1923, Oikawa was assigned the cruiser , followed by the the following year. He then served in a number of staff positions until his promotion to rear admiral on 10 December 1928. In 1930, Oikawa was appointed to the Imperial Japanese Navy General Staff, and in 1932 became Director of the Imperial Japanese Naval Academy. He was promoted to vice admiral on 15 November 1933. Oikawa strongly supported the London Naval Treaty while a member of the Imperial Japanese Navy General Staff, and was thus a member of the Treaty Faction within the navy.