You can burn calories without oxygen (anaerobic metabolism), but it mainly occurs during really intense exercise or loss of blood flow. I think it ends up averaging out in the long run, but I’m not really finding answers at my reading level. At mild exertion levels, the o2-calorie relationship is basically 1:1.

The “true” answer would be a whole-room calorimeter, but thats a much more significant investment of time and resources.