DAO refers to the Depressor Anguli Oris muscle. This is a facial muscle located at the corners of the mouth and is responsible for pulling the corners of the mouth downward, which can contribute to frown lines or a “sad” expression.
When Botox is used in the DAO area, it is typically to relax this muscle and reduce downward pulling, resulting in a more neutral or even slightly lifted appearance of the corners of the mouth.