Significant improvements in the switching voltage distribution are required for the development of unipolar resistive memory devices using MnOx thin films. The Vset of the as-grown MnOx film ranged from 1 to 6.2 V, whereas the Vset of the oxygen-annealed film ranged from 2.3 to 3 V. An excess of oxygen in an MnOx film leads to an increase in Mn4+ content at the MnOx film surface with a subsequent change in the Mn4+/Mn3+ ratio at the surface. This was attributed to the change in Mn4+/Mn3+ ratios at the MnOx surface and to grain growth. Oxygen annealing is a possible solution for improving the switching voltage distribution of MnOx thin films. In addition, crystalline MnOx can help stabilize the Vset and Vreset distribution in memory switching in a Ti/MnOx/Pt structure. The improved uniformity was attributed not only to the change of the crystallinity but also to the redox reaction at the interface between Ti and MnOx.