As an ensign, Ogasawara served in combat during the First Sino-Japanese War on the cruiser "Takachiho" at the Battle of the Yalu River. He was subsequently served on the "Amagi" before transferred to the Navy General Staff Records Department, and assigned to compile an official record of naval operations during the war (which was published in 1903). Ogasawara continued to serve on the General Staff as a naval intelligence officer through the Russo-Japanese War and World War I, rising to the rank of vice admiral by 1918, aside from a one-year tour of sea duty as executive officer of the "Chiyoda" in 1903, and as captain of the cruiser "Tokiwa" and battleship "Katori" in 1912. He was an expert on codes and ciphers, but by the time he had reached flag rank, he was best known as public relations expert for the navy, with his colorful writings on naval history earning him the nickname of the "literary admiral".