Where does oxygen for astronauts come from? Are we just constantly sending rockets with tanks of oxygen up to the ISS?
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That is almost what is done. What is launched to ISS is water that is split into oxygen and hydrogen by electrolysis with power from the solar cells.
It is not just new water that is split but also urine, sweat, and moisture that is breath out are clean and recycled. In our metabolism, we use oxygen both to create carbon dioxide and water so some of the recycled water oxygen comes from the air that is breathed in.
The reason water is launched and not pure oxygen is that is simpler and lighter to store.
Liquid oxygen needs to be cooler than −182.96 °C; −297.33 °F so you need a complex, heavy, and energy-consuming cooling system to do that.
The density of oxygen as gas is quite low so you need high pressure to store a significant amount. High pressure means you need a tank that can handle that and they are heavy. It is not just weight that is a limit when you launch stuff to space, volume is another limitation
Both will expand in volume if heated up or released from a tank, which is a potential hazard in a confined environment like ISS
Liquid water is simple to store and can have the same temperature as is maintained for the crew. Water is 88% oxygen by weight.
The result is launching water is the most cost-efficient, simple, and safest way to deliver oxygen there.
Ther are tanks with pressurized oxygen gas on ISS. So you have a buffet for the produced oxygen and as a backup if there is a problem with the electrolysis. So you might have oxygen for a week stored so you can fix the problem in the electrolysis equipment and in the case of the work leave the station without needing to rush it.
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