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. 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. 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.