Anyone know what Army Regulation (or any branch) that allows a higher-ranking officer to call a lower-ranking officers by their first name?

I think that if a Jr Officer has an issue with their superior addressing them by their first name, they should probably speak up and say something about it, but I personally have never seen it does as a sign of disrespect. Much more the opposite, if that Sr Officer respects their subordinates and values their input is generally when I've seen it occur. I used to work for a full bird that called everyone by their first name. When he met you he would ask your first name and that's what he'd call you the rest of the time you worked for him. He was the only officer that did that with the enlisted as well, but he'd been in so damn long (also prior enlisted) no one ever corrected him on it and it probably wouldn't have been taken very well if they'd tried. We were all actually pretty impressed that he could remember everyone's first name. The confusing part would come in when he couldn't remember a last name and he would keep yelling for someone to find Stefan, but no one had a clue who that was. He's actually the only person in the military to use my first name and not have me say something about it as that is a name that only my wife uses and everyone else just calls me Smitty.

No such regulation.. that higher ranking officer could call his LT's Turnip 1, 2, and 3 if he whishes.. Though it would be unbecoming and unprofessional.

Naw, scary is when your commander tells you to close the door and addresses you by your rank. Then you know you have really messed up.

Take it as a compliment that your superiors care enough to actually know your first name.....

Historically, I do know that the British tradition held that once a man joined the "mess", whether the most junior, or most senior...things were on a first name basis; particularly when in the mess. This was an especially great privilege when extended to a former NCO promoted into commissioned rank, and I imagine it was designed to announce his ascendancy. My guess is that this was because as "gentlemen" within the rigid class structure, you could in all likelihood have a coronet who was a baron...or a colonel who was a commoner. Referring to one another in the familiar probably allowed military hierarchy to remain intact without getting into tricky situations involving alcohol.

In the modern, US context-I always found that people respected "billet" more than rank in most wardrooms. O-1 to O-3 are essentially "equal", with the caveat that an O-3 can hand out an arse chewing to more junior ranks every bit as effectively as more senior ranks. I rarely had a department head (O-4 to O-5) call me by my rank or last name, but then again...I never called them by anything but "Sir" until I reached O-3. Captains (O-6), and I imagine "Full Bird" colonels are able to do just about whatever they want...and while I did have a "first name" relationship with my last CO, I'd have never called him "Gerald" in front of more junior officers, let alone the enlisted ranks.

It matters? Are you pleased to be recognized?? What is the problem?

Would you rather not be noticed?

I saw officers do this my whole career. Honestly, it was never on my radar to worry about.

On submarines, we refer to officers as "Mister". So LT Jones, would be "Mister Jones", Ensign Murphy would be "Mister Murphy," etc. The only exceptions are the Captain and Executive Officer who are "Captain"/"Skipper" and "XO" respectively. When an officer is acting in command, they are referred to as the title, ie; "Officer Of The Deck," "Diving Officer" etc.

I was taught if a Senior Officer calls you by your 1st name that you are on that Officer's good side.

From the IG Desk: Protocol dictates that common military courtesy requires proper recognition of any service member's rank during address at all times. Some Commands relax that somewhat in close quarters...say, a small office. Others may, with permission, extend that somewhat. The key is "permission." If one is uncomfortable being addressed in that manner, then solve the issue at the lowest level by advising the Officer, with respect, that use of proper military courtesy is welcomed (no 'hair on fire' response). If that request isn't honored, then run the issue up the chain. If the "Chain" itself is the problem, see your local IG for resolution.