A change in the charge carrier density of zinc oxide (ZnO) films for control the functioning of thin-film transistors (TFTs) has been studied by oxygen (O2) plasma techniques. This effect was interpreted in terms of a threshold voltage shift and the variation in carrier mobility. The plasmasurface interaction on the molecular level and the behavioral characterization of ZnO films were investigated by X-ray photospectroscopy of the O 1s region. This process was highly sensitive at low level variations in defect and doping density. O2 plasma treatment leads to a shift of turn-on voltage and a reduction of the off-current by more than two orders of magnitude in ZnO-TFTs.