Various electrodes modified with cobalt-porphyrin-clay complexes (CoTMPyP- and CoTAPP-Mon, Ver, and AW-Kao), silver colloid (AgCol), and polymers [PVA and P(2VPy)] were synthesized and their electrochemical properties were investigated to evaluate the possibility of creating oxygen sensors. The oxygen reduction based on the two-electron transfer mechanism (Eq. 1) occurred in the cobalt-porphyrin-clay/AgCol/polymer-modified electrode and the peak current density (I-p) depended quantitatively on the concentration of oxygen. As for the evaluation of oxygen sensors in the modified electrode systems at the optimal composition, (i) the I-p increased linearly with the concentration of oxygen, (ii) the response was a reversible and rapid one, and (iii) the modified electrodes in the Mon and Ver-systems were most stable (lifetime > 6 h under hydrodynamic conditions). The cobalt-porphyrin-clay/AgCol/polymer-modified electrode systems acted effectively as oxygen sensors.