Every office has its own protocol for who is called by his or her first name and who is called by his or her title. New employees should follow suit, after listening carefully to how people are addressed.
If yours is a "title" office, but you call your boss Charlie when meeting alone, you should still call him Mr. Dodd when others are around.
If yours is an informal office, you still should wait for the other person to say, "Please call me Jim," before doing so, if he has been introduced to you as "Mr. Culyer."
Everyone, no matter whether the office is formal of informal, has a name. No assistant should ever be referred to as "my girl." She is, if a possessive must be used, "Charlene Walter, my assistant," or "Angela Badalato, my assistant."