With the start of the Russo-Japanese War, Ōsako participated in the Siege of Port Arthur, under the overall command of General Nogi Maresuke’s Third Army. His forces played a major role in the bloody Battle of 203 Meter Hill, where he also lost his third son. and afterwards fought at the Battle of Mukden. He returned to Japan in March 1906, was awarded the Order of the Golden Kite, 2nd class in April and was promoted to general in May. In September 1907, he was also elevated from baron to viscount ("shishaku"). He entered the reserves in April 1909 and retired from service in April 1914.