This study investigated the origin of dielectric loss induced by O2 plasma on organo-silicate glass low-k dielectrics. The contributions from the polarization components to dielectric constant were delineated by analyzing the results from capacitance-voltage measurement, spectroscopic ellipsometry, and Fourier transform infrared spectroscopy together with the Kramers–Kronig dispersion relation. The dielectric loss was found to be dominated by the dipole contribution, compared with the electronic and ionic polarizations. The origin of the dipole contribution was further investigated by performing quantum chemistry calculations. The physisorbed water molecules were found to be primarily responsible for the dipole moment increase and the dielectric loss.