Owing to its large work function, MoO_X has been widely used for hole-selective contact in both thin film and crystalline silicon solar cells. In this work, thermally evaporated MoO_X films are employed on the rear sides of p-type crystalline silicon (p-Si) solar cells, where the optical and electronic properties of the MoO_X films as well as the corresponding device performances are investigated as a function of post-annealing treatment. The MoO_X film annealed at 100 °C shows the highest work function and proves the best hole selectivity based on the results of energy band simulation and contact resistivity measurements. The full rear p-Si/MoO_X/Ag-contacted solar cells demonstrate the best performance with an efficiency of 19.19%, which is the result of the combined influence of MoO_X’s hole selectivity and passivation ability.