Disclosed is a method of manufacturing an oxide semiconductor device, including: forming a gate electrode on a substrate; forming a gate insulating layer on the gate electrode; forming an active pattern on the gate insulating layer; forming a first mask pattern on the gate insulating layer and the active pattern; forming an insulating interlayer on the gate insulating layer, the active pattern, and the first mask pattern; forming a second mask pattern on the insulating interlayer, the second mask pattern comprising an opening that exposes a region where the first mask pattern is formed; forming contact holes exposing portions of the active pattern by patterning the insulating interlayer using the first mask pattern and the second mask pattern; and forming a source electrode and a drain electrode on the gate insulating layer by filling the contact holes, the drain electrode spaced apart from the source electrode.