ELI5 International Space Station - How does the ISS never run out of oxygen?

Since space has no air in it how can astronauts breathe fresh air inside the International Space Station (ISS)? Do we ship oxygen up to the ISS in tanks? Do we presently have the technology to make a self-sustaining biosphere in space?

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Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: twentythousandleague, happythoughts413, masu12, santi871, mvilla & tico_de_corazon

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