NH₃ can be liquefied at ordinary temperature without the application of pressure. But O₂ cannot, because

Critical temperature is the temperature at and above which vapor of the substance cannot be liquified, no matter how much pressure is applied. It is the parameter that tells about the Van der Waal force of attraction. Higher the Van der Waal force of attraction, higher will be its critical temperature and it would be easy to liquefy the Gaseous State 38 gas. Here, 3 NH has high force of attraction and critical temperature whereas 2 O has low critical temperature and weaker intermolecular force of attraction, so it is not easy to liquefy it.