NO post-oxidation annealing (POA) processes with different temperatures and durations are simultaneously studied on the n-type and p-type 4H-SiC MOS capacitors. The results from Secondary Ion Mass Spectroscopy (SIMS) reveal that nitrogen diffuses further into the SiO2 layer for the samples annealed at higher temperature (1250 °C). Furthermore, the area densities of nitrogen for the n-type samples are about 1.3 times larger than the p-type samples. The densities of negative effective fixed dielectric charge (Neff) and slow-state electron traps are effectively decreased with increasing annealing time and temperature for the n-type samples. However, the introduced nitrogen leads to more the deep energy-level traps [0.5–0.6 eV above the valence band edge (Ev)] in the p-type samples, which can exchange electrons with near-interface traps in response to the alternating current (AC) signal below 10 kHz. Besides, the densities of the slow-state hole traps, which are obviously increased at low temperature but raised slowly at high temperature, are mainly affected by the temperature of the NO POA process and not only the density of incorporated nitrogen. Therefore, NO annealing at high temperature with a short duration (at 1250 °C 1 h) may be a good choice to optimize NO POA treatment for n-channel 4H-SiC MOSFETs, based on comprehensive consideration of the influence of different NO POA conditions on n-type and p-type MOS capacitors.