1:2 is the mole ratio of O:H in water (or 1/3 O, 2/3 H)
The molecular formula of water is H2O.
this means that in every 1 mole of water molecules , there is 1 mole of oxygen atoms and 2 moles of hydrogen atoms .
So the ratio of oxygen to hydrogen is 1:2
or 1 atom in every 3 is oxygen
and 2 atoms in every 3 is hydrogen.