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In this work, we study oxide defects in various III-V/high-k metal-oxide-semiconductor (MOS) stacks. We show that the choice of a given starting measurement voltage with respect to the MOS flat-band voltage affects the observed capacitance-voltage hysteresis. We discuss how this behavior can be used to study the distribution of oxide defect levels. With the help of comprehensive experimental data, we show that Al2O3 and HfO2 have different hysteresis characteristics related to different oxide defect distributions. In case of an Al2O3/HfO2 bilayer stack with Al2O3 on the channel side (interfacial layer, IL), as the IL thickness reduces from 3 nm to 0 nm, the hysteresis behavior switches from the typical Al2O3 behavior to the one corresponding to HfO2. We link the characteristic behavior of two dielectrics to the defect level distributions inside their respective band-gaps through a simple energy-driven charging model. Based on the experimental data and simulation results, we show that Al2O3, despite having a lower peak defect density as compared to HfO2, shows a very wide, almost continuous distribution of defect levels across and around the InGaAs channel energy band gap. These results explain the often reported poor reliability of III-V devices with Al2O3-based gate stacks.