Generally, transition metal oxides are of growing importance in the engineering of catalytic systems and electronic devices. Most metal oxides are wide bandgap insulators, which implies experimental problems in their characterization. Usually charging effects arise when charged particles are applied as analytical tools. In principle, the charging problems can be overcome by using thin epitaxial layers of metal oxides prepared on metal substrates. Ag and Au are suitable substrates for CoO thin films because of the relatively small misfit of about 4.3 %. The CoO layers were prepared by Co deposition in O2 atmosphere on Ag(100) and Au(111). The nucleation and growth of epitaxial CoO layers have been studied at substrate temperatures between 300 K and 600 K by scanning tunneling microscopy and low-energy electron diffraction at 300 K. Additionally, for identification of the oxidic films, characterization of their electronic structure and to optimize the imaging conditions for atomically resolved results, the dependence of scanning tunneling microscopy images on the sample-bias voltage has been investigated. The dependence of the step height of CoO can be explained qualitatively by contributions of the tunneling current into of from oxide states, and correlates with the width of the optical bandgap of the oxide.