An organic light emitting display device, comprising: a substrate; and a pixel on the substrate, the pixel including: a driving thin film transistor (driving TFT), the driving TFT including a poly-Si active layer formed of poly-silicon; a switching thin film transistor (switching TFT), the switching TFT including an oxide semiconductor active layer formed of an oxide semiconductor material; and an organic light emitting diode (OLED) electrically connected to the driving TFT; wherein an interlayer insulation layer formed of a first material including hydrogen is provided between the poly-Si active layer and the oxide semiconductor active layer, and wherein a gate insulation layer of the switching TFT is provided between the interlayer insulation layer and the oxide semiconductor active layer, the gate insulation layer formed of a second material different from the first material, the second material being configured to block diffusion of hydrogen from the interlayer insulation layer to the oxide semiconductor active layer, and the first material includes silicon nitride SiNx, and the second material includes silicon oxide SiOx, characterized in that the gate insulation layer includes a first gate insulation layer and a second gate insulation layer disposed on the first gate insulation layer, wherein a density of the second gate insulation layer is higher than a density of the first gate insulation layer, the second gate insulation layer being closer to the oxide semiconductor active layer than the first gate insulation layer. a substrate; and a pixel on the substrate, the pixel including: a driving thin film transistor (driving TFT), the driving TFT including a poly-Si active layer formed of poly-silicon; a switching thin film transistor (switching TFT), the switching TFT including an oxide semiconductor active layer formed of an oxide semiconductor material; and an organic light emitting diode (OLED) electrically connected to the driving TFT; wherein an interlayer insulation layer formed of a first material including hydrogen is provided between the poly-Si active layer and the oxide semiconductor active layer, and wherein a gate insulation layer of the switching TFT is provided between the interlayer insulation layer and the oxide semiconductor active layer, the gate insulation layer formed of a second material different from the first material, the second material being configured to block diffusion of hydrogen from the interlayer insulation layer to the oxide semiconductor active layer, and the first material includes silicon nitride SiNx, and the second material includes silicon oxide SiOx, characterized in that the gate insulation layer includes a first gate insulation layer and a second gate insulation layer disposed on the first gate insulation layer, wherein a density of the second gate insulation layer is higher than a density of the first gate insulation layer, the second gate insulation layer being closer to the oxide semiconductor active layer than the first gate insulation layer. a driving thin film transistor (driving TFT), the driving TFT including a poly-Si active layer formed of poly-silicon; a switching thin film transistor (switching TFT), the switching TFT including an oxide semiconductor active layer formed of an oxide semiconductor material; and an organic light emitting diode (OLED) electrically connected to the driving TFT; wherein an interlayer insulation layer formed of a first material including hydrogen is provided between the poly-Si active layer and the oxide semiconductor active layer, and wherein a gate insulation layer of the switching TFT is provided between the interlayer insulation layer and the oxide semiconductor active layer, the gate insulation layer formed of a second material different from the first material, the second material being configured to block diffusion of hydrogen from the interlayer insulation layer to the oxide semiconductor active layer, and the first material includes silicon nitride SiNx, and the second material includes silicon oxide SiOx, characterized in that the gate insulation layer includes a first gate insulation layer and a second gate insulation layer disposed on the first gate insulation layer, wherein a density of the second gate insulation layer is higher than a density of the first gate insulation layer, the second gate insulation layer being closer to the oxide semiconductor active layer than the first gate insulation layer.