We study the charge transport mechanism of electron via traps in thermal SiO2 on silicon. Electron transport is limited by phonon-assisted tunnelling between traps. Charge flowing leads to oxygen vacancies generation, and the leakage current increases. Long-time annealing at high temperatures decreased the leakage current to initial values due to oxygen vacancies recombination with interstitial oxygen. Taking into account results of ab initio simulations, we found that the oxygen vacancies act as electron traps in SiO2.