An oxide semiconductor thin film transistor and a flat panel display device incorporating the same oxide semiconductor thin film transistor. The thin film transistor includes a gate electrode formed on the substrate, a gate insulating layer formed on the substrate and covering the gate electrode, an oxide semiconductor layer formed on the gate insulating layer and covering the gate electrode, a titanium layer formed in a source region and a drain region of the oxide semiconductor layer, and source and drain electrodes respectively coupled to the source region and the drain region through the titanium layer and made of copper. The titanium layer reduces the contact resistance between the source and drain electrodes made of copper and the oxide semiconductor layer, forms a stable interface junction therebetween, and blocks a diffusion of copper.