Air becomes liquid at -190°C
Air is mainly nitrogen, oxygen and carbon. You can separate these components of air by slowly cooling it to -200 centigrade. At -79 C (CO2 boiling point) the carbon dioxide will become liquid and drop to the bottom where you can separate it.
Then cool it to -200C and the oxygen and nitrogen will turn to liquid. Next, in a fractionating column, gradually heat the liquid to -196C (Nitrogen boiling point) and the Nitrogen will gradually rise to the top and can be drained away, leaving you with pure oxygen.