遼陽会戦
During the Russo-Japanese War, Ōshima was commander of the IJA 3rd Division under the Japanese Second Army, under General Oku Yasukata. He led the division at the Battle of Liaoyang, Battle of Shaho, and the Battle of Mukden. At the end of the war, he was promoted to general, and served as Governor-General of Kwantung Leased Territory from October 1905 to April 1912. During this time, he laid the foundations of what would be called the Kwantung Army.