 Herein, we report the electrochemical water oxidation efficiency of nano-Co3O4 catalyst samples obtained by the thermal decomposition of Co(NO3)2·6H2O at various temperatures (320, 420, 520, and 620 °C). The structural and morphological details of the synthesized samples were determined by employing X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. These studies revealed the formation of well-dispersed nano-Co3O4 particles with sizes, shapes, and crystallinity levels that differed for the different decomposition temperatures. The prepared catalysts were immobilized on filter-paper-derived carbon electrodes for checking their electrochemical properties. The electrochemical efficiency levels of the Co3O4 catalyst samples were evaluated by employing each of them as an anode to study the water oxidation reaction. The nano-Co3O4 sample prepared at 420 °C yielded the highest efficiency and good stability towards the water oxidation reaction. The higher efficiency of this sample was attributed to the relatively small average size and low level of agglomeration of its nanoparticles, and to the high electrochemically active surface area of its electrode.