A thin film transistor includes a gate electrode; an active layer formed of an oxide and insulated from the gate electrode; and a source electrode and a drain electrode formed of an oxide on the active layer such that the source electrode and the drain electrode are insulated from the gate electrode and electrically connected to the active layer, wherein the active layer, the source and the drain electrode are formed using an atomic layer deposition (ALD) and an insitu process, and a root mean square (RMS) value of the surface roughness of the active layer which contacts with the source and drain electrodes is less than 1 nm in order to reduce the contact resistance between the active layer and the source and drain electrodes, a method of manufacturing the same, an organic light emitting display apparatus including the thin film transistor, and a method of manufacturing the same.