Ōshima was born in Iwamura Domain, Mino Province (currently part of Ena City in Gifu Prefecture) as the son of a samurai household.

He graduated from the 4th class of the Imperial Japanese Army Academy in 1881, where he specialized in artillery and was sent overseas as a military attaché and for studies to France and Prussia from 1891 to 1893.

Ōshima served on the staff of General Yamagata Aritomo during the Japanese First Army in First Sino-Japanese War.

During the Russo-Japanese War, he served on the Imperial General Headquarters, and after the war held various administrative and staff positions within the Imperial Japanese Army General Staff.

[1] He entered the reserves in 1919 and served as a member of the House of Peers in the Diet of Japan from 1920.