In this research, we investigated transparent zinc-indium-oxide (ZIO)-based thin-film transistors (TFTs) using an organic gate dielectric layer on a glass substrate. The channel layer was deposited at room temperature by using an rf magnetron sputtering system with a ZIO target and Ar/O$_2$ as a sputtering gas. The electrical properties of the TFTs varied with the O$_2$ partial pressure when the channel layer deposition was carried out. With a bottom contact-type TFT geometry, using Au as the source-drain electrode, the ZIO-based TFT showed a field-effect mobility of up to 0.5 cm$^2$/Vs and on/off ratio of more than 10$^5$.