Ion-beam-sputtered indium-tin oxide (ITO) films were studied as a function of the oxygen pressure PO2, during deposition. Analysis of electrical transport measurements, Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS) show a self-consistent correlation of all the results. With increasing PO2 a monotonous decrease is observed in the carrier density, which is found to be directly proportional to the oxygen vacancy concentration. This is based on a direct evaluation of the concentration of In unoxidized species in the film using AES and XPS. The moderate decrease of the electron mobility as PO2 is increased is also attributed to the change in the film oxidation state. The systematic relations between all the parameters investigated in this study yield a better understanding of the deposition process and point in the direction of achieving the best ITO films.