system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where does oxygen for astronauts come from? Are we just constantly sending rockets with tanks of oxygen up to the ISS?
gpt
It's pretty self sufficient.  There's no way they could keep running up everything they need constantly so the ISS recycles a lot.

Oxygen can be made through electrolysis.  This is where using electricity you can turn water into oxygen and hydrogen.  The hydrogen can then be combined with the carbon dioxide the astronauts breathe out to turn back into water...... and some methane.

The air filtration systems remove the moisture from the air (from the astronauts breathing and sweating) and recycle it for either drinking or turning into air.

Water is also reclaimed from the waste that the astronauts produce.  Shower water is also reclaimed.  

This severely limits any resupply runs they may need.