Abstract The performance of an improved atom superposition and electron delocalization molecular orbital (ASED-MO) model has been investigated for the ground and selected low-lying excited states of the entire sequence of the highly-polar cationic and anionic first-row transition-metal oxide diatomics. In particular, in both series of compounds deep potential energy curves of an almost Morse-type shape were derived allowing for an adequate calculation of the equilibrium geometries, bond dissociation energies, as well as other spectroscopic constants. With some exceptions, values derived are in good agreement with both the experimental and theoretical ones, where available, whereas in the case of the anions most values derived are predictive theoretical ones appearing for the first time. The variation of the MO bond strenghhs, ionization potentials, electron affinities and hardness along the sequences of the cationic and anionic species follow “double humped” pattern, characteristic of many periodic properties of the transition metals. The calculated ground states for both series of compounds are also discussed. Present results, along with our earlier findings for the neutral MO molecules, provide a further confirmation of the good performance of our ASED-MO variant in the case of transition-metal oxide diatomics.