On graduation, Oikawa was promoted to lieutenant commander, and was appointed aide-de-camp to Crown Prince Hirohito in 1915–1922.

After his promotion to captain on 1 December 1923, Oikawa was assigned the cruiser Kinu, followed by the Tama the following year.

While Navy Minister, he strove to maintain ties with the United States, and instructed his naval attachés in Washington DC to work together with the Japanese ambassador to prevent war from breaking out.

Likewise, he strongly opposed suggestions that Japan should declare war on the Soviet Union in early 1941.

[2] He continued to serve as Naval Councilor to near the end of World War II and was Chief of the Navy General Staff in late 1944 but resigned in May 1945 and was replaced by an Ōita native Toyoda Soemu in an attempt to soften down the Imperial Japanese Army's Ōita-born leadership.