A solution-gated indium-tin-oxide (ITO)-based field effect transistor (FET) without interfaces among the source, channel, and drain electrodes, which is called the one-piece ITO-FET, can be simply fabricated from a single sheet of ITO by etching the channel region. The direct contact of the ITO channel surface with a sample solution contributes to a steep subthreshold slope and a high on/off ratio. In this study, we have examined the effects of oxygen vacancies and hydroxy groups at the ITO channel surface on the electrical characteristics of the one-piece ITO-FET. In the transfer characteristics, the turn-on voltage gradually increased over the days of storage in deionized water and then recovered after the heat treatment at 200 °C in vacuum. This resulted from the finding that the densities of oxygen vacancies as a source of carriers and hydroxy groups as charged species at the ITO channel surface changed under each condition, which were analyzed by X-ray photoelectron spectroscopy. In particular, the pH responsivity of the one-piece ITO-FET was improved and maintained with increasing the density of hydroxy groups at the ITO channel surface. Therefore, the control of the densities of oxygen vacancies and hydroxy groups at the ITO channel contributes to the improvement of the detection stability of devices when targets are actually measured.