Why molecules of oxygen must have an even number of water

The question is not appropriate. The correct explanation is given below.

Explanation:

The molecule of the oxygen cannot have the water molecule but the water molecule have the molecules of oxygen, having formula of H2O. A molecule of oxygen is made by combining more than one atom of oxygen forming a molecule as the atoms combines to form a molecule. Also, the presence of number of molecules of a particular element in a compound is fixed according to the law of definite proportion which tells that: The atoms of elements present in a compound are in a fix ratio by mass to be called the molecule of that particular compound. For example: The Formula of water is H2O having 2 atoms of hydrogen and one atom of oxygen forming a ratio of 2:1 to form ONE molecule of water. If this ratio is not maintained then we cannot call it a molecule of water.