Theoretical and experimental methods are applied to investigate the degradation of SiO₂/O₂ gate-stacks in state-of-the-art MOSFETs. A combination of density functional theory and nonequilibrium Green's function formalism has been applied to the atomic scale calculation of the leakage current through SiO₂/O₂ dielectrics. Samples with different dielectric stacks have been taken into account to study the thickness dependence of SiO₂ and HfO₂ on the leakage current. The calculated results show a good agreement with the leakage current and constant voltage stress measurements. The current influenced by oxygen vacancies, particularly in the High-k dielectric close to the SiO₂/O₂ interface has been analyzed. Comparison between the measurement and simulation results show that oxygen vacancy defects in the HfO₂ are a likely cause for progressive stress-induced leakage current in MOSFETs with ultrathin High-k gate-stack.