An oral/nasal cannula manifold gas sampling and oxygen delivering manifold for sampling exhaled breath of a subject and delivering supplemental oxygen, the cannula including a main body portion having a suction port which is connected with a collection tube to a suction device for sampling the exhaled breath of the subject. A nasal prong upwardly protrudes from the main body portion and is positioned for insertion into a nostril of the subject to collect nasally exhaled breath. An oral conduit passage is embedded in a deflector/concentrator plate that extends downwardly from the main body portion and is also provided with a passage with an elongate lateral opening positioned for placement near the mouth of the subject to collect orally exhaled breath of the subject. The conduit passages of the nasal and oral passages are connected to and aligned in the same plane whereby the opposed nasal and oral conduit passages are connected with the conduit passage of the suction port without any adjacent connected void volumes or dead spaces in any of the passages.
