Einstein yes, but he wasn't actually a member of a socialist party, he just supported it personally and wrote arguments defending it.
Oppenheimer, I think not entirely though he was surely sympathetic at least. His wife, his brother, and his best friend were card-carrying communists, but idk of any evidence that he ever espoused socialism/communism himself, outside of hanging out with communists a lot. He was very much accused of being a communist after the war ended, but pretty much every famous person who wasn't a fascist was targeted by mccarthyist witchhunts and accused of being communist