5Day: Most likely, they will introduce themselves to you with their first name only. Thus referring to them by first name in that case would be appropriate. I would never do that but its my military background impacting my actions. In the ROTC setting, they are used to interacting with civilians and will be less formal. That said, it is my experience that USMC officers are more rigid in regards to this. My advice is to refer to USMC officers as Sir/Mam or Captain "Smith" or whatever. The rank is their first name.