The insulator to metal phase transition in NiO is studied within the framework of reduced density matrix functional theory (RDMFT) and density functional theory (DFT).We find that the spectral density obtained usingRDMFTis in good agreement with experiments both for undoped as well as doped NiO.Wefind that the physical description of the hole-doping induced phase transition qualitatively differs depending on whether NiO is calculated within DFT or reduced density matrix functional. In the former case the underlying mechanism of the phase transition is identified to be a rigid shift of chemical potential, while in the latter case a redistribution of the spectral weight drives the transition. These latter results are found to be in good agreement with both experiments and previous many-body calculations.