I thought Oppenheimer turned out to be a Communist. He ran the Mannhattan project. When Ike found out that McCarthy knew about the connection and the Army's role in it, he stepped in to block the investigations and left McCarthy hanging in the wind.
My recollection (admittedly less than perfect) is that Oppenheimer was a suspected sympathizer and/or had several friends who were at least marginally involved, but it was never proved he (Oppenheimer) had an active involvement with the Communist party.
I'll have to re-read a bunch of stuff (most of which I have loaned out to friends, never to be seen again)to get my facts straight.