Energetic oxygen ion bombardment induces densification and enhances the film crystallinity and refractive index of the evaporated MgO thin films. The optimum oxygen ion bombardment energy is 100 eV and its energy shows have the highest crystallinity and optical properties. The oxygen ion bombardment affects the optical properties that are dependent on the structural properties of the MgO thin films. The ion induced secondary electron emission property is enhanced more by the cesium incorporation than by the ion beam assisted depsition. This phenomenon is a combined effect of densification of the structure and the reduction of the surface work function.