Magnetron sputtered nickel oxide thin films deposited on the native oxide of crystalline n-Si(100) wafers are studied in dependence of the substrate deposition temperature (600 °C, 400 °C, 200 °C, and room temperature) using X-ray and synchrotron excited photoemission spectroscopy as well as cyclic-voltammetry under illumination. We show that the substrate temperature during nickel oxide sputtering governs the composition of the pristine NiO_x film and the OER performance. Two dedicated nickel oxide species are found with Ni²⁺ corresponding to stoichiometric NiO while Ni³⁺ indicates an oxygen rich NiO_x (x > 1) phase. With decreasing deposition temperature, the ratio of Ni³⁺/Ni²⁺ in the pristine NiO_x film increases. Information depth dependent synchrotron related photoemission spectroscopy further suggests that oxygen rich NiO_x is found on top of the surface and at the grain boundaries. The OER onset potential improves from 1.55 V to 1.1 V in correlation to an increasing Ni³⁺/Ni²⁺ ratio in the pristine NiO_x film and an increasing emission from a nickel oxyhydroxide phase (h-NiO_x) after photo-assisted cyclic-voltammetry in alkaline solution. Upon electrochemical treatment, a reconditioning process is observed with the formation of h-NiO_x that consists of Ni(OH)₂ and NiOOH, while NiO_x disappears.