This study investigates the influence of reactive sputtering gas composition, specifically the oxygen-to-argon (O2/Ar) and hydrogen-to-argon (H2/Ar) ratios, on the optoelectrical and structural properties of fluorine-doped tin oxide (FTO) and undoped tin dioxide (SnO2) thin films deposited at room temperature (RT). Through systematic variation of O2 and H2 content in the sputtering atmosphere, gas-phase composition is correlated with key performance metrics, including optical transmittance, sheet resistance, carrier density, and mobility, both before and after postdeposition annealing (PDA) at 400 °C in a nitrogen atmosphere. An optimal O2/Ar ratio of 0.3–0.4% achieves the best optoelectrical trade-off in FTO, yielding a minimum sheet resistance (468 Ω/sq) and high mobility (13.7 cm2/(V s)). In SnO2 films, increasing oxygen improves optical transparency but reduces conductivity, while hydrogen incorporation at fixed 1% O2/Ar enhances transparency and lowers sheet resistance in the as deposited state. These effects are attributed to defect passivation rather than changes in oxidation state, as supported by X-ray photoelectron spectroscopy results. Ambipolar conduction observed in the as deposited films transitions to stable n-type behavior after PDA, highlighting the role of thermal treatment. Although RT sputtered SnO2-based films do not yet match the performance of high-temperature grown benchmarks, these findings demonstrate that careful tuning of the sputtering gas composition enables scalable, thermally compatible, and cost-effective fabrication of transparent conducting electrodes and transport layers in photovoltaic applications.