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|Native name||岡 市之助|
28 March 1860|
Yamaguchi prefecture, Japan
|Died||20 July 1916(aged 56)|
|Allegiance||Empire of Japan|
|Service/branch||Imperial Japanese Army|
|Years of service||1881 - 1916|
|Other work||Minister of War|
Oka was born in Hagi in Chōshū domain (present day Yamaguchi Prefecture. He graduated from the 4th class of the Imperial Japanese Army Academy in 1881, and from the 4th class of the Army Staff College in 1888.
Oka served as Vice Commander of the IJA 8th Infantry Brigade, and subsequently as commander of the IJA 20th Infantry Regiment. During the First Sino-Japanese War, he was Chief of Staff of the IJA 1st Division.
After the war, Oka served in various administrative and staff positions within the Imperial Japanese Army General Staff. At the time of the Russo-Japanese War, he was working in the Military Affairs Department. Oka was promoted to major general in 1905. He subsequently commanded the IJA 27th Infantry Brigade and the IJA 29th Infantry Brigade before being promoted to lieutenant general in February 1912, and commander of the IJA 3rd Division in 1913.
- Harries, Meirion (1994). Soldiers of the Sun: The Rise and Fall of the Imperial Japanese Army. Random House. ISBN 0-679-75303-6.
Apr 1914 – Mar 1916