[jabby_jabby]

Yes, going that cold seems silly.

I would imagine that since air is a conglomerate of different gasses, you would just have to figure out what the freezing point is of oxygen, and then every other gas present in the air (y'know, hydrogen, helium, etc etc) then figure out where oxygen stands. Then cool the air until almost the point that oxygen freezes (or liquifies, whatever), remove everything that is already solid or liquified, lower the temperature to the freezing/liquifying point of oxygen, then remove the solid/liquid oxygen.

Easy peasy.

Actually, I am talking out of my ass. Is this for a science project or assignment? Or your own morbid curiosity? Or do you need pure oxygen for something? If it's the latterest, go to a medical supply store and see if you can get some oxygen in tanks. That's probably simplest.