An organic light emitting display having an improved image quality. The organic light emitting display includes a data driver for supplying a data signal to data lines; a scan driver for supplying a scan signal to scan lines; pixels at crossing regions between the data lines and the scan lines; a power source unit for generating a first power; a first voltage divider for dividing the first power to generate a first reference power; a second voltage divider for dividing a power from an external power source to generate a second reference power; and a switch to transmit one of the first reference power or the second reference power to the data driver. Each of the pixels includes an organic light emitting diode that emits light when a current flows from the first power to the second power.