To improve the chemical reaction rate of oxygen charging on a manganese dioxide electrode of a fuel cell/battery system, we used highly dissolved oxygen in an electrolyte. For this chemical reaction, a discharged electrode was placed in a pressure-resistant chamber containing an electrolyte and then oxygen was supplied at 0.5, 1.0, 1.5, and 2.0MPa to the electrolyte for 20, 40, and 60min. Electrochemical characterization and X-ray diffraction were carried out to measure the amount of charge and the changes in the crystal structure before and after the chemical charging process. The results obtained from the chemical charge/electrochemical discharge experiments indicate that the dissolved oxygen in the alkaline solution can chemically charge the discharged electrode. The charge rate and amount were found to increase with O2 partial pressure and supply time. Additionally, X-ray diffraction confirmed that the chemical charging process with dissolved oxygen is accompanied by a crystal structure change with the emission of protons and electrons from MnOOH in the same manner as in the electrochemical charging process.