system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do astronauts aboard the ISS always have an oxygen supply?
gpt
They  have some plants which helps, but they're not really there to generate oxygen.  Most of oxygen is generated by electrolysis, take water and break into hydrogen and oxygen.   Collect the oxygen and vent the hydrogen into space.

resupply runs bring water along with other cargo.  And water is reclaimed from the air and urine of astranauts.