Hydrogen and oxygen, both are elements, no matter whether taken as atoms or molecules(of alike atoms). The Chemists usually prefer to get the amount of any element in a compound in number (found with the help of the Avogadro's number). The relative stoichiometric measures show a significant variation if the relation is changed, e.g. if this mass/mass measure is changed to mass/volume or volume/volume measures, the percentage amount of each of the elements will vary. So, the more general format used by the chemists for the expression of amount of any element in any compound/s is taken in the number of it's molecules/atoms.