was a general in the Imperial Japanese Army, Japanese ambassador to Germany before and during World War II and (unwittingly) a major source of communications intelligence for the Allies. His role was perhaps best summed up by General George C. Marshall, who identified Ōshima as "our main basis of information regarding Hitler's intentions in Europe". After World War II, he was convicted of war crimes and sentenced to life imprisonment, but was paroled in 1955.

Biography Early life Ōshima was the son of a provincial low ranking samurai family from Gifu Prefecture. His father, , rose up through the ranks and eventually served as Army Minister from 1916 to 1918. Much was expected of the young Ōshima and his rise in the Imperial Army structure was swift. He graduated from the Imperial Japanese Army Academy in June 1905, a member of the academy's 18th class, and was promoted to second lieutenant in June 1906 and to lieutenant in June 1908. He graduated as a member of the 27th Army War College class in May 1915 and was promoted to captain the following year.

From 1918 to 1919, he served in Siberia with the Japanese expeditionary forces and was appointed assistant military attaché in the Japanese embassy to the Weimar Republic. Promoted to major in January 1922, he served as a military attaché in Budapest and Vienna in 1923 and 1924. After his return to Japan, he was promoted to lieutenant colonel in August 1926. Following a promotion to colonel in August 1930, he served as commander of the 10th Field Artillery Regiment from 1930 to 1931.Ammenthorp, The Generals of World War II

Military and diplomatic career In 1934, Colonel Ōshima became Japanese military attaché in Berlin. Described as "more German than a German",Coox, Alvin D., Japanese Foreknowledge of the Soviet-German War, 1941. Soviet Studies, Vol. 23, No. 4 (Apr., 1972), pp. 554-572 he spoke the language almost perfectly, and was soon befriended by Joachim von Ribbentrop, then Adolf Hitler's favourite foreign policy advisor. Although Hitler ostensibly used the Foreign Ministry (Auswärtiges Amt) for his foreign relations, he was actually more dependent on the Dienststelle Ribbentrop, a competing foreign office operated by the ex-champagne salesman.

Ōshima was promoted to major general in March 1935. Under Ribbentrop's guidance, Ōshima met privately with Hitler that fall. With the support of the Nazi leadership and the Imperial Japanese Army General Staff, Ōshima progressed rapidly while in Berlin. He attained the rank of lieutenant general and was appointed ambassador to Germany in October 1938. He then transferred to the army reserve. During his early months as ambassador, according to evidence presented later at the Nürnberg Trial of Major War Criminals, he plotted the assassination of Joseph Stalin by Russian agents sympathetic to his cause.Record of conversation between Oshima and Himmler, January 31, 1939, Trial of Major German War Criminals, vol. 2, p. 135. Also Nazi Conspiracy and Aggression, vol. 1, p. 152 In a conversation that Ōshima had with Heinrich Himmler on 31 January 1939, the former expressed the hope that German-Japanese co-operation in the field of intelligence would lead eventually to the disintegration of the Soviet Union.Trial of Major German War Criminals, vol. 3, p. 371. Ōshima was instrumental in the forging and signing of the Anti-Comintern Pact on 25 November 1936 and the Tripartite Pact on 27 September 1940.

However, on August 25, 1939, the German government decided to conclude the Molotov–Ribbentrop Pact and to suspend negotiations on a Japan-German alliance and defense agreement. This caused great turmoil in the Japanese government, contributing to the collapse of the Hiranuma Cabinet. Ōshima was recalled to Japan (with Saburō Kurusu succeeding him) to take responsibility in September 1939, and was dismissed as an ambassador on December 27.

Ōshima's importance for Hitler can be seen in the fact that after the conclusion of the Anti-Comintern Pact, the US Ambassador in Japan, Joseph Grew, estimated that the agreement was mainly the result of Ōshima's work, without the participation of the Japanese Ministry of Foreign Affairs.Joseph Grew to the Secretary of State, December 4, 1936, Foreign Relations of the United States, 1936, vol. I, p. 404. Also Grew to the Sec. of State, January 1, 1937, ibid, 1937, vol. III, p. 7 At the insistence of the Nazi government, he returned to Berlin as ambassador in February 1941, and remained in that position until the German surrender in May 1945. He dedicated his efforts to closer relations between the two countries. This including military cooperation in the Indian Ocean area (in the form of anti-merchant submarine warfare). Such was his fanatical belief in Nazi ideology that American journalist William L. Shirer, in The Rise and Fall of the Third Reich, wrote that Ōshima was "more Nazi than the Nazis."

Ōshima's close relationship with Hitler and Ribbentrop gave him unparalleled access, for a foreigner, to German war plans and national policy, comparable to that of Winston Churchill with the American war leadership. In turn, Hitler admired the militaristic Japanese and made Ōshima a personal confidant.

Ōshima made visits to the Eastern Front and the Atlantic Wall, and he met periodically with Hitler and other Nazi leaders. Being a meticulous military officer in training, he wrote detailed reports of the information provided to him by the Nazis. The reports were sent by radio to Tokyo in the Purple diplomatic cipher. Unbeknownst to the Japanese, the Purple cipher had been broken by American codebreakers in 1940. Thus, Oshima's reports were read almost simultaneously by his superiors in Japan and by Allied leaders and analysts as "Magic" intelligence. Sometimes, the Allies read the reports before the Japanese did, as transmission problems between Germany and Japan often held up the reports for hours. Ōshima was interviewed in 1959 and asked about the security of his transmissions. He said that he had been warned in 1941 by Heinrich Georg Stahmer that there were signs that Japanese diplomatic messages were being read by the Allies, but that he was convinced that, by double-encrypting his dispatches, he had ensured they could not be decrypted by the Allies. He died before the Allied decryption of Purple messages became common knowledge and so never knew that he had unwittingly provided the Allies with priceless intelligence.Bōei Kenkyūsho, Chūō Gunji Gyōsei sono ta 95, ‘Bōchō ni kansuru kaisō chōshuroku’ (handwritten transcripts of interviews with Ōshima Hiroshi conducted in November 1959 by Major-General Kubo Muneharu and others), interview no. 5.Peter Kornicki, Eavesdropping on the Emperor: Interrogators and Codebreakers in Britain's War with Japan (London: Hurst & Co., 2021), pp. 93-95.

Pacific War On 12 February 1941, Ōshima discussed the possibility of a joint German-Japanese initiative for war against the British Empire and the United States with Ribbentrop, agreeing with him the time was ripe to strike at the British Empire in Asia.Doc. PS-1834, text in Nazi Conspiracy and Aggression, vol. IV, pp. 469-475 and Trial of Major German War Criminals, vol. 3, p. 373. On 23 February 1941, Ribbentrop urged him to press the Japanese government to attack British possessions in East Asia.Trial of Major German War Criminals, vol. 3, p. 371. On 28 November 1941, in a conversation with German Foreign Minister, Ōshima was given an assurance that the Third Reich would join the Japanese government in case of war against the United States.Trial of Major German War Criminals, vol. 1, pp. 215-216.

Such was Hitler's high esteem that Ōshima was one of only 15 recipients of the Grand Cross of the Order of the German Eagle in Gold. Hitler awarded the medal following the attack on Pearl Harbor in 1941. The award ceremony was attended by Reich Foreign Minister Ribbentrop and the secret notes of the conference were revealed at the Nuremberg trials in 1945. In addressing Ōshima, Hitler reportedly said:

Germany presses for Japanese attack on Soviets

Despite Ōshima's anti-Soviet positions, the Japanese government in April 1941 concluded a non-aggression pact with Moscow. The German armed forces invaded the Soviet Union in June and the German government was interested in a simultaneous Japanese attack on the USSR. However, prior to the invasion itself, the German government had not updated Ōshima about plans of attack. In a conversation held on 17 May 1941, Ernst von Weizsäcker, State Secretary in the German Foreign Office, denied that there was any tension with the Soviet government.Trials of War Criminals before the Nürnberg Tribunals, vol. 12, p. 165 ("The Ministries' Case")

Following the invasion of the Soviet Union on 22 June 1941, pressure was placed on the Japanese government to join the invasion. On 9 July 1942, Ribbentrop tried to convince Ōshima to urge his government to join the attack on the Soviet Union. Ribbentrop's main argument being that "never again would Japan have such an opportunity as existed at present to eliminate once and for all the Russian colossus in eastern Asia".Trial of German Major War Criminals, vol. 3, p. 386. On 6 March 1943, Ōshima delivered Ribbentrop the following official statement from the Japanese government:

Supporting war crimes Ōshima's high esteem with Hitler made him privy to some of the planning relating to actions later to be defined as war crimes and atrocities. One example was a meeting held between Ōshima and Hitler on 3 January 1942, where they both agreed on the action of sinking life-boats working for the rescue of Allied naval personnel fleeing ships sunk in military action. The official text reads:

At a meeting that Ōshima had on 25 May 1944 with Hitler and Ribbentrop, Hitler advised that the Japanese government should publicly hang every captured US pilot who was involved in air raids in hope of deterring further such attacks.Trial of German Major War Criminals, vol. 10, p. 384.

Oshima was later charged and found guilty for the part he played for the forging and signing of the Anti-Comintern Pact and the Tripartite Pact, which were, in the eyes of the Allies, part of the conspiracy to wage aggressive war.

Intercepted dispatches

Virtually all of Ōshima's dispatches as ambassador were intercepted: approximately 72 during the last 11 months of 1941, some 103 in 1942, 400 in 1943, 600 in 1944, and about 300 during the just over four months of 1945 when Germany was at war.These original statistics were compiled by Carl Boyd, Hitler's Japanese Confidant: General Ōshima Hiroshi and MAGIC Intelligence, 1941-1945 (Lawrence: University Press of Kansas, 1993), pp. 42-43. For example, in a dispatch decoded on 19 January 1942, Ribbentrop agreed to supply daily intelligence reports to Ōshima, which he could pass on to Tokyo. He warned that "any leakage of these reports due to our fault would be of grave consequence, so all the handling of these reports should be strictly secret." This despite the fact that the Germans often reproached him of the unreliability of the Japanese codes, although Ōshima assured them of its security. This laxity proved to be fatal to Japanese espionage efforts, as even much of the intelligence gathered by the Japanese spy network codenamed TO in Spain (with implicit support given by the Spanish authorities) was channelled through him. This evidence halted the loading of petroleum by the United States onto Spanish tankers in 1944.

While some of his predictions were wrong (Ōshima predicted that Britain would surrender to Germany before the end of 1941), his reporting of the Nazi leadership's plans and policies and his factual data were invaluable to the Allies. For example, on 6 June 1941, he advised Tokyo that Germany would invade the Soviet Union on 22 June (see Operation Barbarossa).