Former intelligence officers we interviewed in Moscow stressed that Oppenheimer's assistance was of great importance during the 1942-1944 period.
Oppenheimer is mentioned as an unlisted member of the American Communist Party in a Soviet intelligence document dated January 7, 1946.
https://www.wilsoncenter.org/publicatio ... discussionThe Oppenheimer files in Soviet Intelligence Archives and the Presidential Archives remain under seal. The critical 1944 and 1945 documents in the Soviet history of atomic energy have not been published although earlier and later materials have been released.
The fact that after all this time Oppenheimer's files are still under seal should pretty much confirm what we know... he spied for the Soviet Union.