This paper reports on the conduction mechanisms and trapping effects in SiO2/4H-SiC MOS-based devices subjected to post deposition annealing in N2O. In particular, the anomalous Fowler-Nordheim (FN) tunnelling through the SiO2/4H-SiC barrier observed under consecutive reverse bias sweeps was studied by temperature and time dependent gate current measurements. The excess of gate current with respect to the theoretical FN predictions was explained by a chargedischarge mechanism of Near Interface Traps (NITs) in the oxide. The gate current transient was described with a semi-empirical analytical model, modifying the standard FN model with a time-dependent electric field to account for the neutralization of trapped charges at NITs.