2H₂O $\to$ 4e^- + O₂ + 4H⁺ . The equivalent weight of oxygen is

2H₂O → O₂ + 4H⁺ + 4e^-
In the formation of one mole of oxygen, four mole of electrons are lost
EW of O₂ =$\frac{\mathrm{Mol}.\mathrm{Wt}}{4}$                  

= $\frac{32}{4}=8$