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. During the
Satsuma Rebellion of 1877, he was promoted to
major. After the war, he served in a number of
staff positions with the Sendai Garrison and became a colonel in 1886. In 1887 he became chief-of-staff of the Tokyo Garrison and following the reorganization of the Imperial Japanese Army under the recommendations of
Prussian
military advisor Jakob Meckel, he became chief-of-staff of the
IJA 1st Division. In June 1891, Ōshima was promoted to
major general and was assigned command of the IJA 9th Infantry Brigade, which was also styled the “Ōshima Combined Brigade”. Dispatched to the
Korean Peninsula in 1894 during the
Donghak Rebellion, his 4,000 man force was tasked with expelling the
Empire of China's
Beiyang Army from Korean territory by force. 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. In February 1898 he was promoted to
lieutenant general. He served on the
Imperial Japanese Army General Staff from September 1911, and was awarded the
Order of the Paulownia Flowers in June 1912. He retired from service in August 1915, and died in 1926. ==Decorations==