How Do You Supply Oxygen To Your Colonies?

How do you supply breathable air to your colonies, ships, space habitats, ect.

The only ways I know how to make breathable air are oxidized rocks like on the moon and Mars (apparently the moon has so much of these rocks that air would be a waste product). The other way is electrolysis on water to make oxygen and hydrogen.

One species in my setting called Pthumerians have the latter in their ships and in their new home in Olympus Mons. Machines called Wellsprings are hooked up to subterranean water sources (apparently Mars has an underground ocean so vast it could flood the planet up to a mile). The Wellsprings make oxygen & hydrogen and funnels them through various tubes, the hydrogen has other uses.