The diagnosis, the therapy and the monitoring of the obstructive sleep apnea syndrome (OSAS) are to be considered as critical factors in order to prevent severe systemic complications that can be caused by this pathological condition. This work presents three cases of adult patients affected by OSAS, undergoing therapy with oral appliances (OAs), meant to remove the pharyngeal obstruction caused by mandibular retrusion and tongue collapse in nocturnal supine position. The aim of this work was to evaluate the effect of the OAs both through the comparison of the polysomnographic analysis, in the presence and absence of the appliance in situ, and also through the volumetric measurements of the pharingeal airway thanks to the cone beam computed tomography.