In this work, we study the properties of tin oxide films, which were annealed in oxygen ambient for various periods. The as-deposited tin oxides are tin-dominant and, from the Hall measurements, they are of the n-type with high electron concentrations (>1019cm-3) and would change to the ptype when the oxygen annealing is sufficiently long. We have also found that changes in the structure and crystallinity of the channel layer can be clearly observed by X-ray diffraction analysis and optical microscopy. On the basis of the observations, a physical scheme is proposed to describe the evolution of the electrical performance of oxygen-annealed devices. A hole mobility of 3.24cm2V-1 s-1, a subthreshold swing of 0.43V/dec, a threshold voltage of 1.4V, and an on/off current ratio larger than 103 are obtained as the channel is transformed into SnO.