To describe a method to separate both substances, we look at the properties of of both and take advantage of these properties. For instance, the difference of the boiling point of the substances is very large so we can use this property. We can cool both substances to a temperature lower than 90.2 K (oxygen) but higher than 4.2 K (helium). At this state, oxygen is liquid while helium is in the gas state so we can separate them completely.