First, you will need to create an Oxygen Collector, which likewise requires power to run. In the Overworld, you can place it anywhere to begin gathering oxygen; but in a no-atmosphere environment you will need to surround it either with leaves (which it will slowly consume) or growing wheat (which stops producing oxygen when it's full-grown, so needs to be harvested and replanted regularly.
But in the Overworld, you can place it anywhere. But then you need...
In order to do anything with the oxygen collected by the collector, you will need to create Oxygen Pipes using six panes of glass on the regular crafting grid - three across the top and bottom. You will use the Oxygen Pipe to feed the Oxygen from the Oxygen Collector to the...
You will need to pipe the oxygen into the Oxygen Compressor, which will use it to fill...
You will use the Oxygen Compressor to fill Oxygen Tanks. There are three sizes of oxygen tank that you can build -- the Light Oxygen Tank, the Medium Oxygen Tank, or the Heavy Oxygen Tank.
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