The electron band structure and charge density of MgO are investigated theoretically at V0/V=1.0, 1.25, 1.50. It is found that compression increases the band gap to a value of the order of 10 eV at lower mantle conditions. The prediction of increasing band gap is consistent with the experimentally observed decrease of the index of refraction of MgO. The valence charge density shows a significant amount of delocalization which further increases with compression. Ionic radii and ionicities cannot be uniquely defined, and in particular, the distribution of the valence charge, and therefore the properties of the oxygen 'ion,' are strongly influenced by the nature of the cation. Some geophysically important consequences are a likely increase in the transparency of MgO and its silicates to thermal radiation and a very large electrical resistivity at lower mantle conditions.