Oxygen is like a milder form of nitrogen when it comes to its changing forms.
Oxygen becomes a gas when heated above -183 degrees Celsius, and when it is cooled below -183 degrees Celsius, it becomes a liquid.
Just as a bonus, the transition between liquid and solid forms happens when Oxygen is cooled below -218.79 degrees Celsius.
So your answer is "the conversion between the liquid and gaseous states of Oxygen." :)
