An oral cannula for placement within the oral cavity of a patient for delivery of a treatment gas and for collecting end-tidal carbon dioxide (ETCO2) includes a treatment gas delivery lumen including at least one aperture near a distal end of the treatment gas delivery lumen; an end-tidal CO2 sampling lumen. The cannula also includes a cap having (i) a gas diverter adapted for diverting at least a portion of the treatment gas and (ii) an end-tidal CO2 inlet including flutes and apertures there between that are in communication with the end-tidal CO2 sampling lumen. The oral cannula is adapted for custom bending or has a bend such that the oral cannula is insertable and retainable in a patient's mouth and functional for supplying a treatment gas and sampling gas exhaled by the patient.