A thin film transistor (TFT) using an oxide semiconductor as an active layer, a method of manufacturing the TFT and an organic light emitting display device having the TFT. In one embodiment, a TFT includes a first gate electrode formed on a substrate. A source electrode is formed to be spaced apart from the gate electrode on the substrate. A first insulating layer is formed on the substrate. An active layer is formed of an oxide semiconductor on the first insulating layer, and connected to the source electrode. A second insulating layer is formed on the first insulating layer. A second gate electrode is formed on the second insulating layer so as not to overlap with the first gate electrode. A drain electrode is formed to be spaced apart from the second gate electrode on the second insulating layer, and connected to the active layer.