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 don't know about any regulation, but I know that pretty much all of the Officers in my unit (grade higher or equal) call each other by first name. The Junior Officers use rank and last name, but the others all use first names. I assumed it was a comraderie thing, and that it was normal. I heard it at my last unit as well.

Hahaha, this is so common where I work.