Why do plants breath oxygen at night? explain it and how oxygen helps the plant

The term "breath" is improper, instead of this term you could use the right term "respiration".

As every living organism, plants, during the process of respiration, take in the oxygen and release carbon dioxide.

Also, during photosynthesis, plants absorb carbon dioxide and emit oxygen,photosynthesis being the process used to form the nutritious substances necessary for growth and development of plants, and it took place in the chloroplast only in the presence of light. Chloroplasts contain a green pigment called chlorophyll, which absorbs light energy from the sun.

From water and minerals (crude sap arrived in leaf ) in the presence of carbon dioxide in leaf, organic substances (sap drawn) and oxygen are formed. Starch is the organic matter from the sap drawn, resulting from photosynthesis.
Oxygen is removed from the leaf through stoma.

Knowing all the above, the conclusion is that during the night, the process of photosynthesis, which changes CO2 into O2 to feed themselves, cannot be held, because of the lack of light, so the plants have to use O2 for carrying the electrons.