Why is the inductive effect of chlorine, bromine, and iodine stronger than that of hydroxy group, even though oxygen is more electronegative?
With hydroxyl groups, the oxygen is donating electron density into the ring (resonance). Additionally, the overlap between oxygen and carbon pi orbitals is sufficient enough to make up for the electronegativity of oxygen.
Halogens also donate electron density, however, the orbitals do not overlap as well with halogens and carbon, causing less electron donation then oxygen. Thus, electronegativity trumps resonance.