A quantitative study of the surface composition of clean metallic and partially oxidized nickel exposed to oxygen was carried out employing X-ray photoelectron spectroscopy (XPS). By fitting both branches of the metallic Ni 2p spectrum, it was possible to resolve two previously unreported satellite peaks in the Ni 2p_1/2 portion of the spectrum. For the partial oxidation of the nickel surface, we analyzed the Ni 2p, O 1s, and C 1s angle-resolved XPS spectra. Using state-of-the-art peak-fitting (including the block approach) and background modeling methods (including the active approach) it was possible to discriminate the peaks of Ni₂O₃ from those of metallic nickel for both branches of the Ni 2p spectra from partially oxidized nickel. The oxidation of metallic nickel is through the formation of a Ni⁺³ oxide layer together with deep protrusions. The uncertainties are calculated with the covariance matrix method. The strong overlap and shape similarity of the Ni⁰ L₃M₄₅M₄₅ Auger structure with that of the Ni⁺³ strongly suggests that the modified Auger Parameter does not provide direct additional insight about the chemical state to that provided by the photoemission spectra.