Oxygen plasma resistance of low-k organosilica glasses (OSGs) is shown to be strongly dependent on the material structure: a silicon oxycarbide has a higher oxygen plasma resistance compared to a conventional OSG. The silicon oxycarbide is stable at pressures up to 300 mTorr, which is 10 times higher than those of the conventional OSG. Even at higher pressures, the degradation is much lower. Structural analysis using wet etching demonstrated that the stability at low pressures is due to a thin protecting layer: dense oxide formed by impingement of directional oxygen ions. The inside layer is shown to have the same k-value as the original film. The superior oxygen plasma resistance of the silicon oxycarbide is probably due to lower methyl group content, which provides greater volume reduction toward achieving a dense siloxane networks that protects the inside.