The resistance switching current-voltage (I-V) characteristics in polycrystalline NiO films were investigated in the temperature range of 10K<T<300K. Very clear reversible resistive switching phenomena were observed in the entire temperature range. An analysis of the temperature dependence of the resistance switching transport revealed additional features, not reported in previous studies, that weak metallic conduction and correlated barrier polaron hopping coexist in the high-resistance off state and that relative dominance depends on the temperature and defect configuration. In addition, the authors propose that metallic Ni defects, existing near polycrystalline (or granular) boundaries, play a key role in the formation of a metallic channel.