Measured X-ray charge densities of the alkali-earth oxides, and theoretical calculations of the electronic structure of MgO and CaO, suggest that the metal's valence states play a very significant role in simple oxide's response to pressure. Of particular interest is the effect of compression on d-orbitals or bands, which are responsible for observed metallizations in a number of rare earth chalcogenides, and are predicted to lead to metallization of CaO. The rare earth chalcogenides data suggest that, for a series of compounds in which the anion or cation does not change, metallization pressures are positively correlated with ratio of anion to cation atomic numbers. New calculations of the electronic structure of SrO and BaO produce results that are in agreement with that trend and further confirm the dominant role of d-bands. It is predicted that SrO should become metallic at approximately 3 Mb while the corresponding pressure for BaO is 800-1200 kb, thus allowing for an experimental test with diamond cell techniques.