Solid oxygen is turned into liquid oxygen by a process called liquefaction. Liquefaction is the process of converting a gas into a liquid by cooling it to its boiling point and then compressing it.

To liquefy oxygen, it is first cooled to its boiling point of -183 degrees Celsius (-297.3 degrees Fahrenheit) by using a refrigerant such as liquid nitrogen. Once it has reached its boiling point, the oxygen is then compressed until it turns into a liquid. This can be done using a compressor or by using a high-pressure gas cylinder.

The process of liquefaction can also be used to turn other gases into liquids, such as hydrogen, nitrogen, and helium.