They act like this isn't a settled fucking thing.

Oppenheimer was communist-adjacent when all the hep cats were communist-adjacent and when the communists said "hey help us out" he went "hey you have hot girls" and when the communists said "hey help out the USSR" he said "hey I'm not a joiner."

Oppenheimer was loosely-attached to everything and under near-constant surveillance from like '37 on. At the time, it was only suspect to be communist if you were one of those people who were seriously disappointed that Hoover lost election. After WWII of course it was a hangin' offense, at which point Oppenheimer was decidedly not Communist. Prior to that? Hell yeah he had a communist brother, a communist mistress and gave money to the communists while the FBI watched.

This is all openly and plainly dramatized in the biopic that isn't out right now:

The BBC makes it very clear that Leslie Groves brought Oppenheimer on because (1) he was a talented administrator (2) he could be easily watched day and night as well as isolated from anyone and everyone that Groves didn't expressly bring Oppenheimer into contact with. It also has the advantage of (A) not being Chris Nolan (B) not starring Cillian Murphy.