1 mole of a compound is the mass of that substance in grams equal in magnitude to its molecular mass.
The SI unit is mol. Number of moles of a substance (n) = (Mass of substance in grams)/(Molecular mass of substance) the molecular mass of oxygen is 32u, and therefore 32g oxygen is 1mole of oxygen.
The molecular mass of water is 18u. Therefore, 18g of water make 1 mole of water.