We propose GeOx passivation by plasma post-oxidation for dark-current suppression in a germanium (Ge) photodetector (PD). A GeOx/Ge interface exhibits a significantly lower interface trap density than SiO2/Ge and Al2O3/Ge interfaces. GeOx passivation on a Ni/Ge Schottky diode decreases the dark current under -1 V bias by more than one order of magnitude compared with Al2O3 passivation, which is attributed to the reduction in the surface leakage current. We also evaluated the Ge surface potential to study its effect on the surface leakage current. It was found that the surface leakage is suppressed when the accumulation condition of the Ge surface is enhanced as a result of fixed charges in the passivation layer. Thus, we have revealed the importance of a low interface trap density at the Ge surface and a suitable number of fixed charges in the passivation layer for achieving a low dark current in Ge metal-semiconductor-metal (MSM) PDs. Finally, we have examined the effect of GeOx passivation on a normal-incidence Ge MSM PD. We observed a significant decrease in the dark current in the GeOx-passivated samples, and a dark current of 97 nA under -1 V bias was achieved under the optimal GeOx passivation conditions.