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Threshold voltage and drain current instabilities in state-of-the-art 4H-SiC MOSFETs with thermal as-grown SiO2 and NO-annealed gate oxides have been studied using fast I-V measurements. These measurements reveal the full extent of the instability underestimated by dc measurements. Furthermore, fast measurements allow the separation of negative and positive bias stress effects. Postoxidation annealing in NO was found to passivate the oxide traps and dramatically reduce the instability. A physical model involving fast transient charge trapping and detrapping at and near the SiC/SiO2 interface is proposed.