A method of manufacturing an oxide thin film transistor includes forming a gate electrode on a substrate; forming a gate insulating film the gate electrode; forming an oxide semiconductor layer on the gate insulating film; sequentially forming a lower data metal layer and an upper data metal layer on including the oxide semiconductor layer; forming an upper source pattern and an upper drain pattern by patterning the upper data metal layer by a wet etching; forming a lower source pattern and a lower drain pattern by patterning the lower data metal layer by a dry etching using the upper source pattern and the upper drain pattern as a mask to form a source electrode and a drain electrode; forming a first passivation film on the source and drain electrodes; performing a heat treatment on the oxide semiconductor layer; and forming a second passivation film on the first passivation film.