A series of reduced graphene oxide (rGO) samples were prepared by hydrothermal reduction of graphene oxide (GO) samples, which were obtained by a modified Hummers’ method at oxidation temperatures of 65 °C, 75 °C, 85 °C, and 95 °C. The combined characterizations indicate that the number of C–O bonds in rGO samples increases with an increasing oxidation temperature of GO from 65 °C to 85 °C and then decreases when the temperature is further increased to 95 °C. Most importantly, the rGO obtained from the GO at an oxidation temperature of 85 °C (designated as rGO-85) exhibits a higher response to 5 ppm NO₂ (36.7%) at room temperature than the other three rGO samples, which is attributed to the high content of C–O bonds in the rGO-85 sample. This work suggests that the gas sensing properties of rGO-based sensors can be tuned by changing the number of oxygen-containing groups in rGO.