Exposure to oxygen alters the physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). In particular, oxygen in the ambient may influence the device stability of 2D TMDs over time. Engineering the doping of 2D TMDs, especially hole doping is highly desirable towards their device function. Herein, controllable oxygen-induced p-type doping in a range of hexagonal (MoTe₂, WSe₂, MoSe₂ and PtSe₂) and pentagonal (PdSe₂) 2D TMDs are demonstrated. Scanning tunneling microscopy, electrical transport and X-ray photoelectron spectroscopy are used to probe the origin of oxygen-derived hole doping. Three mechanisms are postulated that contribute to the hole doping in 2D TMDs, namely charge transfer from absorbed oxygen molecules, surface oxides, and chalcogen atom substitution. This work provides insights into the doping effects of oxygen, enabling the engineering of 2D TMDs properties for nanoelectronic applications.