The electronic charge distribution and eigenvalues of B1 and B2 CaO were obtained through a set of self-consistent band structure calculations. The zero pressure electron density is in good agreement with the available X ray determination. The results of the calculations demonstrate that bonding in essentially ionic oxides involves considerable delocalization of the valence electrons. This conclusion is supported by the X ray data and by molecular orbital calculations. A comparison of the electronic structures of MgO and CaO shows that they are consistent with expected trends in ionicity. However, large compression tend to invert the relationship, with predicted compressions at metallization being lowest for BaO and highest for MgO. Pressure induced changes in the bonding of MgO differ significantly from the corresponding changes in the heavier alkali-earth oxides because of the presence in the latter of low-energy d bands. It is suggested that under compression, d bands move down relative to other conductions bands and the valence band. The mechanism that causes this may be quite general and is expected to play a significant role in the optical properties of high pressure transition metal-bearing minerals. In particular, it is possible that this mechanism is responsible for the pressure-induced opacity to thermal radiation.