Oxidation of Ni/Au (5 nm/10 nm) contact to p-GaN layer was performed by O2 plasma in a reactive ion etching system. The structural characteristics of the Ni/Au p-GaN for different oxidation time were investigated by x-ray diffraction (XRD) measurements, using an intense synchrotron x-ray source. The XRD measurements indicated that the grains of nickel oxide polycrystalline in the contact were grown continually when the oxidation time increased in 10 min. However, Au showed amorphouslike and the intensities of Bragg diffraction peaks were hardly changed when oxidation time increased to 10 min. The nickel oxide formed by O2 plasma without sequent thermal annealing did not reduce the specific contact resistance (ρc) to p-GaN, but it took an important role in lowering ρc followed by thermal annealing in N2 at 500 °C for 10 min. Optical transmission spectra confirmed that the nickel was easy to be oxidized and few interdiffusions occurred at the metal interface in O2 plasma ambient. Finally, the mechanism of oxidation on the formation of low ρc ohmic contact was also discussed.