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 didn't know there was an AR that covered that, but it was normal in all units I was in, except school house, where students might be present. My pilot explained it was commander's discretion.

Not in a Reg, but when troops are around, frowned upon. When with other officers of the same rank, acceptable. If I was around and an Officer called another officer by their first name an there were troops around, I would call them out. Officer o officer, no troops, OK

when I was ask, by an officer or fellow enlisted what my first name was I informed them it was !st. Sgt.

I know the AF used to be big on using first names. Jr enlisted to E-7. Don't know if that still happens or not.

Most of my Bosses called me by my first name name both in the Army and in law enforcement. When they were overly formal with me I had my reservations about them. Two of my bosses a now deceased distnguised senior officers, I would have followed in Hell without a second thought!!

Off duty or if no one is around I never cared. I never called a LTC or higher by their first name until I was out of the army and my friends that were officers some how became LTC and COL. Some LTs and CPT that I considered friends I did call by their first names off duty and drinking a few beers.

It's funny you mention that, because I didn't realize how bad it was until I was in the loop on email strings. I find it unprofessional but then again being an officer is a different world compared to enlisted.

I think it's probably an unofficial one-grade-up or one-grade-down, and never in public.

There isn't a specific regulation as far as I know in the Air Force. I always thought it was weird when commanders try to use my first name to make me feel human or whatever.

I do not know of any regulation (US Navy), but if an officer were to address me by my first name, I would reply using his first name. If he didn't like it, he can get professional and use the proper address. Respect is a two way street!