Ōshima was a native of Saga Prefecture. He attended the 3rd class of the Imperial Japanese Army Academy and was commissioned as a second lieutenant in the cavalry in 1893. In 1911 he became commander of the IJA 4th Cavalry Regiment. He was promoted to colonel in September 1912, and in October 1913 commanded the IJA 6th Cavalry Regiment. He was promoted to major general in July 1918 and made commandant of the Cavalry School. In August 1922, Ōshima was promoted to lieutenant general. He was assigned command of the newly-formed IJA 14th Division in March 1926 and entered the reserves shortly thereafter.