Promoted to colonel in 1887, he was chief-of-staff of the IJA 4th Infantry Division from October 1890 and appointed head of the 1st Bureau of the Imperial Japanese Army General Staff Office the following year.

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.

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.

From November 1911, Ōsako replaced Nogi Maresuke as the principal of the Gakushūin Peer’s School, serving until August 1917.