Abstract The development of high-performance p-type monolayer transistors is critical for extending two-dimensional (2D) semiconductors beyond silicon in sub 1-nm technology nodes for complementary metal–oxide–semiconductor (CMOS) circuits. However, insufficient doping and metal-induced gap states (MIGS) during metal deposition lead to high contact resistance, greatly limiting p-type on-state currents. Here, we demonstrate high-performance ballistic p-type monolayer tungsten diselenide (WSe 2 ) transistors by oxygen p-doping. The controllable oxygen doping greatly reduces the number of Se vacancy defects in as-grown WSe 2 , and thus increases the hole mobility from 55 to 218.3 cm 2 V −1 s −1 —approaching the theoretical limit. Moreover, this approach also induces a heavily hole-doped contact region in p-type monolayer WSe 2 devices and decreases the contact resistance from 1869 Ω·μm down to 104 Ω·μm. Consequently, a 16-nm-channel WSe 2 p-type field effect transistor (pFET) delivers a saturation current of up to 1635 μA μm −1 at a drain voltage of −1.2 V and a ballistic ratio as high as 81% at room temperature.