Oxygen post-annealing effects on tin (Sn) doped indium oxide (ITO) film are investigated with various analytical tools as a function of temperature, including in situ XRD, ambient pressure XPS (AP-XPS), and Hall measurement. As the annealing temperature increases up to 200 degrees C under the oxygen pressure of 100 mTorr, the in situ XRD shows the evidence of crystallization of the film while the AP-XPS reveals the formation of oxygen vacancy and Sn4+ states on surface. In addition, the mobility of ITO thin film is increased as the post-annealing temperature increases, supporting the results of both in situ XRD and AP-XPS. The results of angle-resolved XPS reveal that the degree of Sn segregation changes little after post-annealing procedure. Published by AIP Publishing.