Transition-metal dichalcogenides have promising potentials for high-performance electronic and optoelectronic applications, which could be deeply influenced by defects, including native defects and dopants. Experiments to date have frequently reported p-type conductivity in the WSe2 monolayer, but the origin remains elusive. Here, using the first-principles calculations, we systematically investigate the point defects in the WSe2 monolayer and show that: (1) no intrinsic point defect is responsible for the p-type doping; (2) hydrogen interstitials (Hi) are possible sources for n-type conductivity; (3) oxygen substitution of Se (OSe) can greatly promote the formation of adjacent W vacancy (VW), and finally make VW relatively shallow acceptors by forming the defect complex nOSe + VW (n = 1 to 6). Our work reveals that nOSe + VW is the origin of the p-type conductivity in the unintentionally doped WSe2 monolayer, given that O is present throughout the synthesis conditions of WSe2.