Recent shock-wave and diamond-cell experiments have shown that CaO transforms from te B1 to the B2 phase at 60--70 GPa. At pressures above 100 GPa, the shock data exhibit unusually large scatter, including two datum points with densities approximately 4.5% larger than those of the B2 phase. A self-consistent band-structure calculation predicts that the band-gap of CaO vanishes at approximately 2.4-fold compression. At densities corresponding to the anomalous shock-wave data, the predicted band gap is narrow enough to allow thermal excitation of electrons into the conduction band. Thus the large scatter may be due to thermally generated instabilities that are premonitory to a metallic or semi-metallic state of CaO. The density excess of 4.5% is similar to the 4% increase observed upon the metallization of EuO.