[en] The influence of a variable oxygen concentration in HfO_x (x < 2) layers on the forming process and resistive switching of TaN/HfO_x/Ni ReRAM cells is investigated. We demonstrate that resistive switching is possible only for those cells for which the Hf/O ratio in a HfO_x layer corresponds to a narrow range for which x ≈ 1.8. The decrease of oxygen concentration in the oxide layer is shown to lead to, on the one hand, the decrease in the I _ON/I _OFF ratio and, on the other hand, to the elimination of the forming process. The analysis of XPS spectra of HfO_x films for a wide range of compositions revealed that the range of x, for which resistive switching is possible, corresponds to a maximum concentration of the Hf₄O₇ phase. The influence of HfO_x phase composition on resistive switching is discussed. (paper)