Abstract: A reservoir cannula is described that has a static fluidic control structure, in that it does not employ a membrane or other moving parts. Furthermore, the reservoir is open to ambient air instead of being sealed. In use, the reservoir cannula enables storage of oxygen and oxygen-rich gas in a storage chamber as well as in and around the patient's nasal passages and nasopharynx, which enables high volume oxygen delivery to the patient early in the next inhalation. Consequently, patients using this delivery mode can carry a smaller and lighter portable oxygen container for ambulatory oxygen, because lower flow oxygen is required to meet their oxygenation needs. In addition, patients requiring a higher flow of oxygen can achieve oxygenation levels previously achieved only by high flow mask or high flow nasal oxygen systems.