The source for the poisoned apple story is Oppenheimer himself, and otherwise uncorroborated to be clear. He spent his life clearly racked by feelings of inadequacy, guilt and self-doubt.

When combined with a somewhat paradoxical large ego and occasionally fanciful reshaping of his own life story or exaggeration, it's entirely plausible (if not likely) that this was in reality a brief intrusive thought or a partially realized fantasy blown up into a catchy anecdote that better fit his self-image of being unable to control his typically human qualities of anger and envy.

If it was Sam Altman, we'd have heard the story from the guy he tried to poison, who instead of filing a police report thought it showed Sam was a real go-getter and offered him his first job on the spot as VP at the company he founded (later forced out by Sam replacing him as CEO, but still considers him a friend with no hard feelings).