This letter reports on the electrical properties of SiO2/4H-SiC interfaces after post-oxidation annealing (POA) in N2O and POCl3. The notably higher channel mobility measured in 4H-SiC MOSFETs subjected to POA in POCl3 was ascribed both to a reduction of the interface traps density and to an increase of donor concentration incorporated in SiC. Scanning spreading resistance microscopy on a SiC surface directly exposed to POA revealed that the incorporation of P-related shallow donors upon POA in POCl3 is more efficient than N-shallow donors incorporation during N2O treatments which subsequently explains the significantly enhanced channel conductivity of the MOSFETs.