Abstract The defects in oxygen plasma irradiated Si have been studied. It is found that two kinds of defects E1(Ec-0.46 eV) and E2 (Ec-0.04 eV) are generated in the sample. The deep level optical spectroscopy reveals that defects E2 has a strong electron-phonon coupling, its Frank-Condon shift is 0.76 eV. For E1, the coupling is weaker, its Frank-Condon shift is 0.04 eV. The analysis shows that the phonon modes coupling to defects E1 and E2 is 28.7 meV and 20 meV respectively.