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

No sir, I don't know but it was common when I was in 84-04. I also heard some 1SGs and SGMs call Lts by their first name also. But never in or around formations.

Chief - you know there isn't and the fact this question is being presented this way is a quintessential demonstration of Warrant Officer smugness. The fact that you're asking this, this way, telegraphs that it bothers you and that you're asking to invoke argument.

Officers just do this. There is no regulation that says this is OK. There's also no regulation which dictates how to hold a fork as long as it gets food in your mouth, no regulation that dictates what brand of watch to wear as long as it keeps time and keeps you on time, and no regulation that dictates which leg to put in your trousers first, as long as you get them both in there, pull them up and wear them decently IAW Army Uniform Regulations. In the course of leading subordinates and communicating with them respectfully, if a Senior refers to a subordinate incidentally by first name... it isn't going to break the Army, destroy morale, or undermine regulations. It's just irritating you.

The fact you took the time to write about this and make it an issue tells me you have too much time on your hands and should worry less about the interpersonal communications of Officers.

Yeah US Air Force. All jokes aside, I won't get into any discussions of psychology but SFC Ward may be on to something. As long as you can maintain the line of respect and the integrity between the individuals involved there shouldn't be any issues. I gave my soldiers permission to call me by my first name both on and off duty. They never did, something about that made for a better working environment. Respect went both ways.

Even as a colonel, I addressed most junior soldiers by rank and name. The exceptions were the occasional friend, also in uniform, but when doing so, they knew they could address me casually as well.

Regardless of Serving, the practice of being on a first name basis with any subordinate, whether or not you are in the same “Chain of Command”, is known as FRATERNIZING or FRATERNIZATION, punishable under the UCMJ.

General's privilege.

During my 20 years this is one of the things that just burned my soul. Even heard officers call enlisted by their first name. Spec ops is the worst at it. I know cause I spent my time in Forse Recon and Bn Recon. Seals are the worst about it. Everyone wants to be like them but not me. Keep some discipline people.

Choosing to refer to a subordinate by different means is a valuable leadership tool and can improve their reception of the message you are attempting to send. Calling a subordinate by first name, nick name, rank and last name, rank only, other (i.e. more colorful name) communicates as much as the actual words. SFC Ward is spot on regarding the "Psychology of Leadership". I would add that communication (method, timing, and message) is a fundamental component of effective leadership.

It’s called leadership. Seniors can take those liberties when they want.

There is no regulation that allows that, it would be ridiculous to dictate such a lapse in military protocol... there are, however, regulations covering the addressing of personnel by rank as the default setting. I've found that most of us were addressed by our last names more often than anything else, except among our 'drinking-level' buddies.

That being said, familiarity in the workplace does foster certain allowances that are generally accepted, working in close proximity with people tends to get somewhat casual as time goes by. It's tough to address your personal staff formally by rank & last name every time you open your mouth without sounding like some unapproachable by-the-book hard-on! I once worked for the S-4 shop (Bn Police Sgt) and our Bn Cmdr was "Sir" or "Lt Col" outside his office, if you went to him with a personal problem, he was "Bob" once his door closed behind you.

Last names are more the norm; There are guys I served with over 35 years ago who's first names I didn't know til they friended me on facebook!