Recently, 2D materials have been intensively investigated for their novel nanoelectronic applications; among these materials, tungsten diselenide (WSe 2 ) is attracting substantial research interest due to its high mobility, sizable bandgap, and ambipolar characteristics. However, Fermi-level pinning (FLP) at the metal-semiconductor contact is a critical issue preventing further integration of WSe 2 to complementary metal-oxide-semiconductor (CMOS) technology. In this study, a facile doping method of oxygen (O 2 ) plasma treatment and an aging effect to overcome the FLP of WSe 2 field-effect transistors (FETs) are utilized. After aging, a reduction is observed in FLP on oxidized WSe 2 FETs, along with a decrease in pinning factor (S) for holes from −0.06 to −0.36. Further, the field-effect mobility of high-(Pd) and low-(In) work-function contacted WSe 2 devices indicates the presence of more improvement in highwork-function metal-contacted p-type WSe 2 FETs, which further strengthens the Fermi level de-pinning behavior attributed to the O 2 plasma and aging processes. The existence of different tunneling behaviors of Pd and In devices also confirms the effect of O 2 plasma doping into WSe 2 FETs. Ultimately, this work demonstrates a simple and efficient method for achieving the de-pinning of Fermi-levels and modulating FLP of 2D FETs.