A method of manufacturing a thin film transistor having a compound semiconductor with oxygen as a semiconductor layer and a method of manufacturing an organic light emitting display having the thin film transistor include: forming a gate electrode on an insulating substrate; forming a gate insulating layer on the gate electrode; forming a semiconductor layer including oxygen ions on the gate insulating layer, and including a channel region, a source region, and a drain region; forming a source electrode and a drain electrode to contact the semiconductor layer in the source region and the drain region, respectively; and forming a passivation layer on the semiconductor layer by coating an organic material, wherein a carrier density of the semiconductor layer is maintained in the range of 1E+17 to 1E+18/cm3 to have stable electrical property.