Oshima'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 Oshima a personal confidante.

Oshima 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 — and promptly reported by radio to Tokyo in the Purple diplomatic cipher. Unknown to the Japanese, the PURPLE code was broken by American codebreakers in 1940; thus Oshima's reports were being read almost simultaneously by those who had access to Magic intelligence. Often, they were able to read them before the Japanese did, as transmission problems between Germany and Japan often held up the cables for hours.

Another example was in November 1943, when Oshima was taken on a four-day tour of the Atlantic Wall fortifications on the coast of France. 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, as well as its manpower and weaponry. He described tank ditches in detail, armament of turrets located close to the shore, and available mobile forces. This provided valuable intelligence to the planners of the D-Day assault. Connected to this was that the Allies knew that Operation Fortitude was working because just one week before D-Day, Hitler confided to Oshima that while the Allies might make diversionary feints in Norway, Brittany and Normandy, they will actually open up "an all-out second front in the area of the Straits of Dover". Thus Oshima dutifully reported that the bulk of German forces would not be waiting in Normandy, but mistakenly, at the Pas-de-Calais area.

His dispatches also proved to be valuable to those who were involved in the bombing campaign in Europe, as Oshima provided details on the effect of Allied bombing raids on specific German targets, giving valuable and relatively unbiased bomb damage assessments to the Allies.