Born in Kokura (in present-day Kitakyūshū) to a samurai family of the Kokura Domain in Buzen Province, Oku joined the military forces of the nearby Chōshū Domain during the First and Second Chōshū expeditions and the Boshin War in their struggle to overthrow the Tokugawa shogunate and bring about the Meiji Restoration.

[1] Appointed a commander of the new Imperial Japanese Army, Oku fought against the disgruntled samurai insurgents during the Saga Rebellion of 1871.

He was elevated to the title of danshaku (baron) under the kazoku peerage system in 1895, and was promoted to army general in 1903.

[4] Oku refused to attend strategy and staff meetings, and thereby gained a reputation for being both a "lone wolf" and also a brilliant tactician capable of independent action.

However, Oku's reluctance to attend the staff meetings was due to his partial deafness, and inability to comprehend and contribute to the discussions.