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. Assigned to the IJA 4th Infantry Regiment, he was promoted to captain the following year, and became battalion commander of the IJA 1st Infantry Regiment in 1873.