We report our first-principles calculations that clarify the electronic states of oxygen-related defects in the 4H-SiC bulk and on the 4H-SiC surface and how they are affected by the surface amorphous structure due to oxidation. It is experimentally reported that thermally oxidized 4H-SiC contains an abundant amount of single-photon sources on its surface (surface SPSs) and that their emitting wavelengths have variance. However, the microscopic mechanism is not clarified yet. In our work, we demonstrate that the energy levels of the oxygen-related defects on the surface are altered sensitively by the local atomic structure of the amorphous surface leading to variations in the wavelengths.