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".

With the support of the Nazi leadership and the Imperial Japanese Army General Staff, Ōshima progressed rapidly while in Berlin.

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.

[3] 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.

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.

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 made visits to the Eastern Front and the Atlantic Wall, and he met periodically with Hitler and other Nazi leaders.

[9] 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.

[10] 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.

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.

But if one sees that the other is interested only in putting one off, in shaming and humiliating one, and is not willing to come to an agreement, then one should strike as hard as possible, and not waste time declaring war.

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.

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".

[14] 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.

The official text reads: the Führer pointed out that, however many ships the United States built, one of their main problems would be the lack of personnel.

[17] 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.

Upon his return to Berlin, he wrote a detailed 20-page report of his visit, giving an account of the location of every German division and its manpower and weaponry.

Ōshima subsequently accepted that order then sent his wife to Bad Gastein, a mountain resort in Austria, and, the next day, left to join her, together with most of the Japanese diplomatic staff.

When brought before the International Military Tribunal for the Far East, he was found guilty of conspiring to wage aggressive war on 12 November 1948 and sentenced to life imprisonment.

After his release, Ōshima lived in seclusion in Chigasaki, Kanagawa, refusing invitations from the ruling Liberal Democratic Party to enter into politics, in stark contrast to Kichisaburō Nomura, who was Japanese ambassador to the United States at the outbreak of World War II and who ran, was elected, and served several terms in the Diet after the war besides active public and private life.