Reset-first resistive random access memory (RRAM) devices were demonstrated for off-stoichiometric Ni_1−xO thin films deposited using reactive sputtering with a high oxygen partial pressure. The Ni_1−xO based RRAM devices exhibited both unipolar and bipolar resistive switching characteristics without an electroforming step. Auger electron spectroscopy showed nickel deficiency in the Ni_1−xO films, and X-ray photoemission spectroscopy showed that the Ni³⁺ valence state in the Ni_1−xO films increased with increasing oxygen partial pressure. Conductive atomic force microscopy showed that the conductivity of the Ni_1−xO films increased with increasing oxygen partial pressure during deposition, possibly contributing to the reset-first switching of the Ni_1−xO films.