Provided are a method of manufacturing a thin film transistor (TFT) substrate and a method of manufacturing an organic light emitting display apparatus, which increase the capacitance of a capacitor without increasing the probability of short circuits between wires. The method of manufacturing a TFT substrate includes (a) forming a capacitor electrode and a gate electrode on a substrate having a first region and a second region, so that the capacitor electrode is formed to correspond to the first region and the gate electrode is formed in a portion of the second region; (b) forming an interlayer insulating layer to cover the gate electrode and the capacitor electrode; and (c) etching a portion of the interlayer insulating layer in the first region by using a halftone mask to a thickness that is less than a thickness of a portion of the interlayer insulating layer in the second region.