[1] Ōsako was born to a samurai family in Satsuma Domain (present day Kagoshima Prefecture).

Ōsako was appointed battalion commander in the IJA 1st Field Artillery Regiment in February 1894.

In June 1901, Ōsako was promoted to major general and commander of the IJA 2nd Field Artillery Brigade.

With the start of the Russo-Japanese War, Ōsako participated in the Siege of Port Arthur.

[1] After the end of the war, Ōsako was awarded the Order of the Golden Kite, 2nd class, and was reassigned to the Inspectorate General of Military Training from February 1906.