Solution-processed oxide semiconductors (OSs) used as channel layer have been presented as a solution to the demand for flexible, cheap, and transparent thin-film transistors (TFTs). In order to produce high-performance and long-sustainable portable devices with the solution-processed OS TFTs, the low-operational voltage driving current is a key issue. Experimentally, increasing the gate-insulator capacitances by high-k dielectrics in the OS TFTs has significantly improved the field-effect mobility of the OS TFTs. But, methodical examinations of how the field-effect mobility depends on gate capacitance have not been presented yet. Here, a systematic analysis of the field-effect mobility on the gate capacitances in the solution-processed OS TFTs is presented, where the multiple-trapping-and-release and hopping percolation mechanism are used to describe the electrical conductivity of the nanocrystalline and amorphous OSs, respectively. An intuitive single-piece expression showing how the field-effect mobility depends on gate capacitance is developed based on the aforementioned mechanisms. The field-effect mobility, depending on the gate capacitances, of the fabricated ZnO and ZnSnO TFTs clearly follows the theoretical prediction. In addition, the way in which the gate insulator properties (e.g., gate capacitance or dielectric constant) affect the field-effect mobility maximum in the nanocrystalline ZnO and amorphous ZnSnO TFTs are investigated.