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.[1]