A thin film transistor substrate and an organic light-emitting diode display including the same are disclosed. In one aspect, the TFT substrate includes substrate and a TFT located on the substrate. The TFT includes a lower gate electrode, a first insulating layer covering the lower gate electrode, an oxide semiconductor layer located on the first insulating layer, a first electrode located on the oxide semiconductor layer and having an island shape, a second electrode located on the oxide semiconductor layer and surrounding the first electrode, a second insulating layer at least partially covering the oxide semiconductor layer; and an upper gate electrode located on the second insulating layer. The oxide semiconductor layer includes a first region, a second region surrounding the first region, and a third region interposed between the first and second regions.