Metal oxide (MO) films (ZnO and CuO) were synthesized by hydrothermal methods and treated with hydrogen and oxygen plasmas. From uv-visible transmittance spectra, we have found that the optical band gaps of MO films blue-shifted with hydrogen plasma treatment, but red-shifted with oxygen plasma treatment. By alternating the treatment sequence of hydrogen and oxygen plasmas, the MO optical band gap values can be reversibly tuned with the tunable ranges as wide as 80 and 550 meV for ZnO and CuO, respectively. The mechanism for reversible tuning of optical property is proposed based on the results of optical emission, X-ray diffraction, and scanning electron microscopy characterization. Compared to conventional metal ion doping and high temperature annealing methods, the use of low-temperature hydrogen and oxygen plasmas is more environmentally friendly.