Abstract: A patient interface including a vent structure that allows a continuous flow of gases exhaled by the patient from an interior of a nasal plenum chamber to the ambient. The vent structure is sized and shaped to maintain the therapeutic pressure in the nasal plenum chamber in use. The vent structure includes a housing secured to the nasal plenum chamber, and a vent wall coupled to the rigid housing and selectively movable between a closed position and an open position. The vent wall is biased toward the closed position. The vent wall is maintained in the closed position and is configured to be positioned in the open position as a result of contact with an oral cushion in order to accommodate the oral cushion with an oral plenum chamber and form an oro-nasal cushion.