He graduated from the Naval Academy in 1911, and from the Naval Staff College in 1923. He began his career in the Navy as a submariner, but he also held various assignments between the two wars. Before the war broke out in the Pacific, he was vice-admiral and political and military specialist in the Navy Ministry.
In October 1940, Takasumi Oka was promoted Rear Admiral and became Chief of the naval affairs bureau of the Navy Ministry, position he kept until July 1944. As such, he exerted influence over the Liaison Conference at which the policy of Japan was partly decided. He is also alleged to have participated in the formation and execution of the policy to wage aggressive war against China and the Western powers.
Takasumi Oka is said to have opposed the Tripartite Pact, between the Axis powers. Later, he would also have supported the withdrawal of Japanese troops from China in order to appease the United-States.
As an admiral, he is alleged to have ordered his men to kill shipwrecked allied.
He supported the idea of Japan attacking the main allied bases in Hawaii and Australia. He also negotiated the support of the army for the Port Moresby operation which led to the battle of the Coral Sea.
Takasumi Oka was arrested at the end of the war and indicted by the International Military Tribunal for the Far East.
