A number of surface analysis techniques, including XPS, UPS and HREELS, are used to study oxygen adsorption on GaP(111) surfaces.Initial oxygen adsorption is found to be saturated at an exposure around 104L. Information concerning surface electron structure variation in this stage, such as surface band bending, chemical shifts, etc. has been obtained. It is found that this stage of oxygen adsorption is related to surface defects. Proper annealing treatment can remove the surface defects along with the adsorbed oxygen. Such oxygen adsorption followed by annealing might be a way for defect removal in surface engineering.