Naive idealist? Useful idiot? Conscious fellow traveler? Hidden party member? Spy?
During the 1920's and 30's, the blurring of the lines was deliberate. The CPUSA was legal and aboveground. The espionage apparatus was secret, but the chiefs wanted to make smoke to camouflage the spies: "nobody here but good American liberals, and anyone who says otherwise is a Reds-under-the-bed conspiracy theorist. And still today, writers on the left often throw up the dupes, useful idiots and fellow travelers to deflect attention from the spies and conflate it all into a diatribe about McCarthyite Red hunting.
I've always been agnostic about Oppenheimer. Fund reports that the Venona intercepts may implicate him as having been a secret Party member, at least for a time. Many of his associates, including his wife, mistress, and brother were party members; he swam in that sea. That doesn't make him a spy, but it is relevant to the question of his security clearance, given that he had lied to the FBI about several details of his communist acquaintances.
To be a communist party member or unenrolled supporter in that era means that one was willing to be a spy. I am hard put to imagine that Oppenheimer never volunteered nor was ever called upon to spy for the Soviets.