Prana, chi, energy....life force. Oxygen is just the vehicle. This is why there's so much emphasis on the breath in meditation and some eastern cultures/religions. There are stories of people who lived in places sealed off from air, but because prana cannot be contained or blocked through physical means, they survived. Also I've read that this is how extremely spiritual people know that death is near because even though they are breathing, prana is no longer entering their bodies; instead it is leaving. They can feel the change.
This is just my abc 123 answer. I'm sure others will go further in depth.
