For an efficient high-temperature reversible solid oxide fuel cell (RSOFC), the oxygen electrode should be highly active for the conversion between oxygen anions and oxygen gas. CaMnO_3-δ(CM) is a perovskite that can be readily reduced with the formation of Mn³⁺ giving rise to oxygen defective phases. CM is examined here as the oxygen electrode for a RSOFC. CaMn_0.9Nb_0.1O_3-δ (CMN) with Nb doping shows superior electric conductivity (125 S cm^-1 at 700 ºC) to CM (1-5 S cm^-1 at 700 ºC) in air and is also examined for comparison. X-ray diffraction (XRD) data show that CM and CMN are compatible with the widely used yttria-stabilized zirconia (YSZ) electrolyte up to 950 oC. Both materials show a thermal expansion coefficient (TEC) close to 10.8-10.9 ppm K^-1 in the temperature range between 100-750 ºC, compatible with that of YSZ. Polarization curves and electrochemical impedance spectra for both fuel cell and steam electrolysis modes were investigated at 700 ºC, showing that CM presented a polarization resistance of 0.059 Ωcm² under a cathodic bias of -0.4 V while CMN gave a polarization resistance of 0.081 Ω cm² under an anodic bias of 0.4 V. The phase stability up to 900 ºC of these materials was investigated with thermogravimetric analysis (TGA) and variable temperature XRD.