We report the optical in situ monitoring results of the laser molecular beam epitaxial growth of complex oxide thin films using an oblique-incidence reflectance difference (OIRD) technique. The sub-monolayer sensitivity, optical interference oscillations, monolayer response observed for heteroepitaxy, and the surface kinetics study of OIRD measurement are presented. By using a four-layer stack model, the interference oscillations and the monolayer response are reproduced with Fresnel's formulas for multi-layers.