In this study we investigated the effect of a NiO x layer on the electrical properties of oxidized Au/NiO x/p-GaN ohmic contacts. Au/NiO x layers with a small concentration of oxygen in NiO x were deposited on p-GaN by reactive DC magnetron sputtering and annealed in a mixture of O 2 + N 2, and in N 2. Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling in combination with transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE SEM) and the circular transmission line method (CTLM) of contact resistance measurements of the contact structure with low content of oxygen in the NiO x layer have been used to explain the reduction of the contact resistance as a result of its anneal treatment. It has been found that creation of a metal/p-NiO/p-GaN contact structure due to annealing of the Au/NiO x/p-GaN structure in either N 2 or O 2 + N 2 is the main mechanism that is responsible for the ohmic nature of the system. However, lowering of the contact resistance is similarly affected also by Ga atoms leaving the vacancies at the metal/p-GaN interface after diffusion of Ga into the metallic layer. The effect of various ways of cleaning the p-GaN surface prior to metallization on the contact resistance has also been investigated.