The electrical characteristics such as threshold voltage, saturation mobility, and electrical reliability of low temperature (350 °C) solution-processed zinc tin oxide (ZTO) thin film transistors (TFTs) were improved considerably by employing O2 plasma treatment. O2 plasma treatment was performed by plasma asher and causes preferential dissociation of weak halide-related bonding such as Cl bonding by ion bombardment. After O2 plasma treatment, the threshold voltage decreased from 25.0 to 10.7 V because of an increase in electron concentration due to the reduction of Cl bonding and simultaneous composition of O-related bonding. The saturation mobility was increased from 0.09 to 0.58 cm2 V−1 s−1 because of the increase in electron concentration and reduction of the halide residues such as Cl atoms acting as trap states by employing O2 plasma treatment. Moreover, the electrical reliability such as threshold voltage shift was improved from 5.34 to 3.23 V for positive gate bias-stress because O2 plasma reduced the halide residues such as Cl atoms acting as trap states.