In Stargate SG-1 S7E3 "Fragile Balance", we see a close-up of Jack's United States Air Force identification card of some sort which clearly indicates his name is "O'NEILL, JOHN J." But basically every wiki article out there about the character indicates that his name is "Jonathan".
- https://www.gateworld.net/wiki/Jack_O%27Neill
- https://stargate.fandom.com/wiki/Jonathan_J._O%27Neill
- https://en.wikipedia.org/wiki/Jack_O%27Neill
Is that correct? As far as I can tell, Jack's ID here indicates that his legal name is "John". If his legal name was actually "Jonathan", that is what would be on his military ID. Yes, most people who go by the nickname "John" are actually named "Jonathan" as their full legal name, but that is not always the case. There is no reason to assume that someone's military ID has a nickname printed on it, right? If someone's military ID indicates that their name is "John", I think it is safe to assume their full legal first name is just "John".
As far as I can tell, there is no indication in the show as to what Jack's full legal name is other than this quick closeup of his USAF ID. The only time the name "Jonathan" is ever actually spoken in Stargate SG-1 is Kinsey clearly talking to his driver in S5E10 "2001". That seems to just be a coincidence, for all we can tell. The name "John" is spoken a few times, but again, always clearly about some other person.
This concept of a legal "John" (not "Jonathan") going by "Jack" is a fairly common occurrence. Think JFK, Jack Nicholson, Jack Welch, Jack Donaghy, etc. Not as common as a legal "Jonathan" going by "John", but it happens. Jack O'Neill actually having the legal name "John" is definitely not unprecedented.






