Oxygen-covered Ni(110) as well as thin NiO films grown by oxidation of that surface were characterized under ultrahigh vacuum by using low-energy electron diffraction (LEED), X-ray photoelectron (XPS), Auger (AES), and ion scattering (ISS) spectroscopies together with CO temperature-programmed desorption (TPD) titrations. Results from NiO(100) films with surface defects induced via Ar + ion irradiation at room temperature were compared with those from partially oxidized ordered Ni(110) surfaces. CO TPD proved to be a useful local probe for the investigation of defective NiO surfaces, since its adsorption energy varies by over 20 kcal/mol in going from a metallic Ni(110) clean surface to NiO. The CO-probing experiments also revealed that Ar + bombardment of thin NiO films leads to the formation of Ni-O phases similar to those found during the early oxidation stages of the Ni metal surface.