After the end of the war, Ōsumi returned to the Naval War College, emerging as a lieutenant commander on 29 September 1906. After serving in a number of staff positions, Ōsumi was assigned as naval attaché to Germany from 27 January 1909 to 1 December 1911.
From 1 December 1918 – 1 July 1921, Ōsumi was appointed as military attaché to France. During that time, he was a participant in the Japanese delegation to the Versailles Peace Treaty negotiations. Also during this period, on 1 December 1920, he was promoted to rear admiral.
His second term as Minister of the Navy was from January 1933-March 1936, during the cabinets of Prime Minister Saitō Makoto and Keisuke Okada. Ōsumi, despite his reputation as a liberal, supported the decision to withdraw from the League of Nations and also argued forcefully for higher naval appropriations budget and re-negotiation of the Washington Naval Treaty. In a "guns and butter" debate, Ōsumi told Japanese legislators that it was incumbent to expand Japan's navy, and that "the whole Japanese nation must make up its mind to cope with the situation, even if we are reduced to eating rice gruel.".[2]
On 7 February 1934, he was awarded the Order of the Rising Sun, 1st class. On 26 December 1935, he was ennobled with the title of baron (danshaku) under the kazoku peerage system.
During the attempted coup by a faction of the Imperial Japanese Army in February 1936 (the February 26 incident, Ōsumi's actions were remarkably ambiguous. Although the commander of the Combined Fleet, Admiral Sankichi Takahashi ordered his battleships in Tokyo Bay and targeted the rebel positions, and the commander of the Yokosuka Naval District, Admiral Shigeyoshi Inoue organized a land force to march on Tokyo, Ōsumi refused to issue any orders or take any action, despite word that Prime Minister Okada Keisuke had survived the attack. After the suppression of the coup, Ōsumi resigned as Minister of the Navy, and served as Naval Councilor from 1936 onwards. In 1940, on the retirement of Prince Fushimi Hiroyasu, Ōsumi became the most senior admiral in the Imperial Japanese Navy; however, he was bypassed for promotion by Osami Nagano.
Ōsumi was killed in action in the Second Sino-Japanese War during an inspection tour of the front lines on 5 February 1941, when his plane, an Imperial Japanese Airways transport, was shot down by Chinese guerrillas soon after takeoff from Guangzhou on a flight towards Japanese-occupied Hainan.[3] He was posthumously awarded the Order of the Rising Sun: Grand Cordon of the Paulownia Flowers. His grave is at the Aoyama Cemetery in Tokyo.
