It's apocryphal. Apparently he told at least one friend at the time that he had done it. Whether he actually did it or just thought about it and was being a dramatic young man will never be known.

https://people.com/j-robert-oppenheimers-grandson-says-oppenheimer-poison-apple-scene-isnt-true-really-serious-accusation-7566508#:~:text=The%20book's%20recording%20of%20the,about%20it%20two%20months%20later.&text=%E2%80%9CHe%20had%20kind%20of%20poisoned,book%20quotes%20Fergusson%20as%20saying.