An organic light-emitting display device, which may be configured to prevent moisture or oxygen from penetrating the organic light-emitting display device from the outside is disclosed. An organic light-emitting display device, which is easily applied to a large display device and/or may be easily mass produced is further disclosed. Additionally disclosed is a method of manufacturing an organic light-emitting display device. An organic light-emitting display device may include, for example, a thin-film transistor (TFT) including a gate electrode, an active layer insulated from the gate electrode, source and drain electrodes insulated from the gate electrode and contacting the active layer and an insulating layer disposed between the source and drain electrodes and the active layer; and an organic light-emitting diode electrically connected to the TFT. The insulating layer may include, for example, a first insulating layer contacting the active layer; and a second insulating layer formed of a metal oxide and disposed on the first insulating layer.