Ultra-thin, p-type conducting Nickel oxide (NiOx) thin films, obtained by room-temperature sputter deposition under an optimum ratio of Argon and Oxygen, is shown to yield a large, nearly temperature-independent, positive shift in the threshold voltage of GaN/AlGaN depletion-mode high electron mobility transistors (HEMTs). The resulting metal-oxide-semiconductor-HEMTs (MOSHEMTs) exhibit a low subthreshold swing of 62 mV/dec, with a low specific ON resistance of 0.37 mOhm.cm2 , a large drain-current ON-OFF ratio of ~ 108, a saturation output current of 333.11 mA/mm at a gate voltage of -0.5 V and a maximum transconductance of 135 mS/mm, whereas the gate-leakage current-density (~  10 microA /cm2) is among the lowest reported so far. The large band-gap (4 eV), yielding large valence (-1.8 eV) and conduction band (1.7 eV) offsets at the NiOx-AlGaN interface, is responsible for the suppression of gate leakage, while the small thickness of the film allows enhanced gate-control of the MOSHEMT channel. These performance metrics are highly encouraging for realization of enhancement-mode GaN/AlGaN MOSHEMTs, with a recessed gate structure.