We studied protons attached to bridging O atoms in the vicinity of the Si(100)-SiO2 interface through density-functional calculations for realistic interface models. These protons do not disrupt the bonding network except in the case of strained SiO bonds for which they lead to the formation of positively charged threefold coordinated Si(3)(+). Defect energies mainly fall within a band of similar to 0.5 eV, which is stabilized by similar to 0.3 eV at the interface, the energies of the Si(3)(+) defects lying at the bottom of this band. Hence, this work indicates that bridging O atoms in the vicinity of the interface can act as shallow proton traps and describes a mechanism for bond-rearrangements leading to Si(3)(+). These results are consistent with experimental observations.