Why do plants breathe in oxygen at night?

that's not true....like all living organisms, plants breathe in oxygen at all times..its just that they involve in photosynthesis , in addition, during daytime

Plants do indeed take in carbon dioxide and expel oxygen. It is the only reason we are here. Oxygen is unstable and must be constantly replenished, without the plants putting off oxygen we could not survive. It is the basic formula for life and is what we search for in the cosmos for other inhabited planets.