Ōshima was born as the eldest son to a samurai of Chōshū Domain (present-day Yamaguchi Prefecture), and fought as a member of the Satchō Alliance forces in support of Emperor Meiji during the Boshin War against the Tokugawa shogunate.

After the Meiji Restoration, he attended military school in Osaka in 1870 and was commissioned as a lieutenant in the fledgling Imperial Japanese Army in August 1871.

[1] On July 28, 1894, his forces defeated the Chinese at the Battle of Seonghwan outside of Asan, south of Seoul in the first land engagement of the First Sino-Japanese War.

For his victory, Ōshima was made a baron (danshaku) in the kazoku peerage system, and assigned to command of the Tsushima Garrison.

[2] He served on the Imperial Japanese Army General Staff from September 1911, and was awarded the Order of the Paulownia Flowers in June 1912.