Photodiodes are key components in opto-electronic systems. Two-dimensional (2D) rhenium diselenide (ReSe2) offers new prospects in such devices due to its thickness-independent, moderate, and direct p type bandgap. We show here that the physisorption of oxygen plays an instrumental role in chemical doping of the ReSe2, as it enhances the positive charge carrier conductivity. We characterized the electrical and opto-electrical responses of a few-layered ReSe2 and ReSe2/hexagonal boron-nitride (h-BN) heterojunction under different conditions of gating and illumination. While the h-BN encapsulation did not change the excitonic nature and atomic bonds of the ReSe2, it prevented its oxygen physisorption, and as a result, considerably deteriorated its conductivity. On the other hand, ReSe2 photo detectors that physiosorbed ambient oxygen experienced enhancement of their conductivity and, therefore, showed improved performances with high currents, excellent responsivity, detectivity, and external quantum efficiency. Thus, this work sheds light on the fundamental physics of ReSe2. We manipulated the conductivity of ReSe2 via its oxygen doping, thereby emphasizing the importance of ambient exposure to the successful operation of ReSe2 as a high-end opto-electronic component.