In H2O molecule, the oxygen is sp3 - hybridised. Two half-filled sp3 - orbitals form O – H s-bonds, while the other two contain lone pairs of electrons. The expected ∠HOH is 109.5°, but the experimental value is 104.5°. This is because lp-lp repulsion > bp-bp repulsion.

On the other hand, H2O2 is a non-planar molecule. The dihedral angle between two planes is 111.5° and ∠OOH is 94.8°. (Gas phase)