Monolayer molybdenum disulfide is considered an extremely promising two-dimensional material for innovative electronics due to its direct bandgap and high charge-carrier mobility. The optical and electronic properties of monolayer MoS2 can, however, be strongly influenced by the specific synthesis route, posing challenges for industrial-scale production. In this study, we investigated the effects of moderate temperature thermal treatments under a controlled O2 atmosphere on the properties of monolayer MoS2 flakes. We found that the treatments can effectively tune the doping level of monolayer MoS2. Notably, 225 °C was identified as the optimal temperature for enhancing its optical emission properties. Our findings suggest that the removal of sulfur vacancies and impurities underlies these effects, demonstrating a promising approach for tuning the properties of monolayer MoS2 at mild temperatures.