This paper investigates the effect of an oxygen (O-2) plasma treatment on multi-layer tungsten diselenide (WSe2) field-effect transistor (FET) by forming tungsten trioxide (WO3) layers. Palladium (Pd), which is known to form an Ohmic contact with WSe2, is used for the source and drain (S/D) contact electrodes as a control group for metal variables. And then, Nickel (Ni), which is thought to form a Schottky contact with WSe2 experimentally, is used as an experimental group. For both cases of the control group and the experimental group, the electrical characteristics including drain current (I-D), on/off ratio (I-ON/I-OFF), subthreshold swing (SS) and field effect mobility (mu(eff)) are analyzed according to the presence or absence of WO3. In case of adopting the WO3 contact layer between the WSe2 and the Ni for the S/D contact electrode, we observe a remarkable improvement in I-D, I-ON/I-OFF, mu(eff), and SS compared to the case without the WO3 contact layer. The analyzed electrical characteristics show that an efficient hole-injection contact was achieved for the multi-layer WSe2 FET by the O-2 plasma treatment, which leads to the formation of an Ohmic-like contact at an electrode/WSe2 interface.