We investigate the initial stages of time-dependent dielectric breakdown (TDDB) in high-field stressed Si/SiO2 MOSFETs via electrically detected magnetic resonance (EDMR). As anticipated, we find that the defects dominating the initial stages of TDDB include silicon dangling bonds at the (100) Si/SiO2 interface (Pb0 and Pb1 centers). We find that the densities of these defects increase with stress time. With similar stressing and optimized measurement temperature, we do observe EDMR of generated oxide defects known as E′ centers. The results indicate that the initial stages of TDDB in the Si/SiO2 system involves a rate limiting step of tunneling between a silicon dangling bond and an oxide defect. Additionally, we have made near-zero field magnetoresistance spectroscopy measurements, which show clear differences with stressing time; these differences are almost certainly due to a redistribution of hydrogen atoms in the oxide.