Baron Oka Ichinosuke (岡 市之助, 28 March 1860 – 20 July 1916) was a lieutenant general in the Imperial Japanese Army and Minister of War during World War I. Oka was born in Hagi in Chōshū domain (present day Yamaguchi Prefecture as the second son of a samurai retainer of the Mori clan.

During the First Sino-Japanese War, he was on the staff of the IJA 1st Division, which was engaged in the brunt of combat operations during that conflict.

During his tenure, he was steadfastly uncompromising in his demands for an increased Army budget to raise two new infantry divisions for Korea, and strongly rejected arguments by Finance Minister Wakatsuki Reijiro that unbridled military expenditures was bankrupting the nation.

On then other hand, Oka unexpectedly opposed his mentor Yamagata Aritomo in many political issues, siding with Okuma Shigenobu and Kato Takaaki against the Rikken Seiyūkai.

[1] He resigned in March 1916 due to ill health, and was ennobled with the title of baron (danshaku) under the kazoku peerage system just before his death in July of the same year.