Cobalt oxide thin films were deposited on glass and silicon substrates using reactive cathodic radio-frequency sputtering from a pure cobalt target in oxygen/argon gas mixtures with different percentages of oxygen (10, 30, 40 and 50%). The X-ray diffractograms showed that deposited thin films crystallized in a cubic Co3O4 phase. The crystallite size was found to vary with the oxygen pressure, reaching a maximum using 40% of O2. Scanning electron microscopy (SEM), AFM, energy dispersive X-ray spectroscopy and Raman scattering analyses confirmed the formation of Co3O4 films with nano-sized particles. In addition, it is found that the optical band gap of the films increased with increasing oxygen pressure in the sputtering atmosphere. Finally, the electrical resistivity of the sputtered thin films was shown to increase with oxygen-percentage.