The enthalpy change at 298 K for decomposition is given in following steps :

Step-1: ${\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\to \mathrm{H}\left(\mathrm{g}\right)+\mathrm{OH}\left(\mathrm{g}\right)\mathrm{\Delta }H=498\mathrm{kJ}/{\mathrm{mol}}^{-1}$

Step-2: $\mathrm{OH}\left(g\right)\to \mathrm{H}\left(g\right)+\mathrm{O}\left(g\right)\mathrm{\Delta }H=428\mathrm{kJ}/{\mathrm{mol}}^{-1}$

Then, value of mean bond enthalpy of $\mathrm{O}-\mathrm{H}$ bond will be: