Carbon paper (CP) is one of the most promising metal-free gas-diffusion electrodes for fabricating sustainable energy-conversion devices such as metal-air secondary batteries using the oxygen reduction reaction (ORR) and CO₂-reforming systems via the electrochemical CO₂ reduction reaction (CO₂RR). The common counterpart reaction for ORR and CO₂RR is the oxygen evolution reaction (OER) from water. The low-overpotential OER catalyzed by nanoparticles (NPs) immobilized on CP has attracted attention for integration in energy-conversion devices. Earth-abundant and low-cost 3d transition metals have been intensively explored as OER catalysts. In this study, we have successfully prepared Fe-Co Prussian blue analog (PBA) NPs with the formulae of Fe_1-xCo_x[Fe(CN)₆]_0.67·nH₂O and Fe_1-yCo_y[Co(CN)₆]_0.67·mH₂O, and the Fe-Co PBA NPs are dispersed into water by surface modification using [Fe(CN)₆]⁴⁻. By a simple drop-coating method, the Fe-Co PBA NPs are immobilized on CP and thermally decomposed into Fe-Co oxides at 400 °C. The OER overpotentials of the Fe-Co oxides are gradually decreased by increasing the Co composition ratios, and the lowest overpotential is 0.46 V vs. RHE at 10 mA/cm² in y = 0.6.