Oroantral fistula  is a communication between the oral cavity and the sinus.  It usually occurs in dental claims when an upper tooth has been removed leaving a hole which leads into the sinus.  The sinus is known for having a thin floor walls and is close the upper back teeth.  A number of clients I have dealt with have only known that they have suffered with OAC after taking a drink of a liquid and that liquid then runs out through their nose! Many dental claims surround the allegation that the dentist has used too much pressure during tooth extraction causing OAF.