Freeze it out.

Breathing comes to mind.

Electrolysis of water. By Chemical reaction. Distillation of liquefied gasses.

01) cryogenic: (Thanks to Uncle Linde of Germany) Application of extremely low temperature as low as -178 to-182 deg cel.Air is sucked from the atmosphere, compressed to 7 to 8 Kg/cm2, then cooled to a tempt of 2 deg cel so that all the natural moisture is condensed. TYhen air is made to pass through a bed of molecular sieve(sieves are excrueted having 1.2mm dia & length 1 to 1.5 cm)molecular sieves absorbs all the CO2 present in the air(water & CO2 having low freezing point, may block the fine path/passages of crogenic section exchanger, because thes will solidify easily, so these two has to be removed).now the air is ready to go into the distillation column.The tempt of the bottom of the column is -160 deg cel, O2 rich gases rise above & reflux rich in the nitrogen comes from top ,these two comes in contact of each other,O2 being less volatile than nitrogen will liquify first(the boiling point of N2= –195.79° C (-320.42° F), and oxygen at –182.96°C cel & will be collected in the bottom of the column.(low tempt is created with the help passing of impure nitrogen vapor through the expansion turbine)O2 purity will you believe = 99.999% 2)Physical process: Atmospheric air is made to pass through Zeolite,which selectively absorbs Nitrogen & allows O2 to pass-purity 95-98%.