I'm writing a historical fiction. For context, it's about Robert Oppenheimer and how he swapped lives with a modern-day kindergarten teacher.
Here's a little piece of dialogue between him and his bestie Ernest Lawrence:
Lawrence tossed the apple into the sparkling lake. "Come on, Oppie, you know you're not supposed to be here."
Oppenheimer turned to his friend, the annoyed expression back on his face. "I've told you so many times, Ernest, and I'm not going to say it again. I can't go back. The kids need me!"
Since our characters call each other by their first names (and nicknames), does that mean I (the third-person omniscient narrator) can do that too? Or since they're historical figures, do I just refer to them by their last names?