The influence of auto-doped oxygen on the activation efficiency of the Si dopant in GaN is directly observed. The activation efficiency of the Si dopant showed a drastic decrease, from 55% to 30%, in response to a slightly increased oxygen concentration. This decrease revealed that auto-doped oxygen (similar to 1016/cm3) in GaN played a crucial role in the activation efficiency of the Si. The detailed compositional, electrical characterization, and photoluminescence results illuminated the oxygen dependence of the electrical properties of Si-doped Hydride Vapor Phase Epitaxy GaN.