The dc oxygen-discharge cleaning of optical elements contaminated by synchrotron radiation has been studied for a variety of contamination levels, optical materials, electrode and reflecting-surface configurations, discharge powers, and exposure times. Cleaning with a proper electrode-surface configuration resulted in the removal of contaminant layers within 1 h even for highly contaminated elements. The reflectance of mirrors and the diffraction efficiency of gratings were almost completely restored after cleaning over the 4-40- eV energy range; the enhancement even amounted to 1 order of magnitude in some cases. This cleaning procedure did not cause any increased roughness; rather, it resulted in a reduced scattering level of contaminated elements. The removal of contaminants also led to a restoration of the grating blaze; essentially, this contributed to an improvement in the inside first-order efficiency around the blazed energy. From the results with controlled power and exposure time, it was shown that an optimum exposure is achieved within several minutes after the disappearance of visible contamination.