What is the most economical way to make oxygen, provided that no chemical method is used, the equipment is very light, small, low power consumption, and a purity of 80-99 is sufficient.

Air is pressurized and liquefied - with the pressure reduced appropriately is separated by nitrogen gassing

What's left is about 95% liquid oxygen

I forget the exact number of atmospheres, but look it up if you're interested

This is supposed to be the lowest cost oxygen