Methods of manufacturing an oxide semiconductor thin film transistor are provided. The methods include forming a gate on a substrate, and a gate insulating layer on the substrate to cover the gate. A channel layer, which is formed of an oxide semiconductor, may be formed on the gate insulating layer. Source and drain electrodes may be formed on opposing sides of the channel layer. The method includes forming supplying oxygen to the channel layer, forming a passivation layer to cover the source and drain electrodes and the channel layer, and performing an annealing process after forming the passivation layer.