Rare earth metal oxides (REOs) represent promising gate dielectric materials for field-effect transistors due to their high dielectric constants required for suppressing leakage currents in devices. In this paper, we present an approach to deposition of uniform REO gate dielectric coatings by oxidation of thin rare earth metal films in air at relatively low temperatures (100–250 °C). The proposed methodology was extensively explored in organic field-effect transistors (OFETs) using five benchmark organic semiconductors and six REOs as gate dielectrics. It was shown that OFETs with the REO gate dielectrics deliver on average considerably better characteristics compared to the reference devices using electrochemically grown aluminum oxide dielectric (AlOx). OFETs on a flexible plastic substrate were demonstrated, which makes this approach very attractive for developing flexible and wearable electronic devices with improved performance.