旅順港閉塞作戦
After his return, Ōsumi was promoted to lieutenant, and served as chief navigator on the cruisers and , and the patrol ship "Manshu" during the Russo-Japanese War. While on "Matsushima", he participated in the Battle of Port Arthur and other combat engagements. He was awarded the Order of the Golden Kite, 5th class on 1 April 1906.