He graduated from the 4th class of the Imperial Japanese Army Academy in 1881, where he specialized in artillery.
He was sent overseas as a military attaché and for studies to France and Prussia from 1891–1893. 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.
He was promoted to major general in 1907 and to lieutenant general in 1913.
From 30 April 1916 to 29 September 1918, Ōshima was Minister of War under Prime Ministers Ōkuma Shigenobu and Terauchi Masatake.
He entered the reserves in 1919 and served as a member of the House of Peers in the Diet of Japan from 1920. From 1940-1946, he was a member of the Privy Council.