An organic light emitting display includes a display unit divided into a plurality of fields (regions), data and scan drivers, a power supply, and a driving voltage calculator. The display unit has a plurality of cathode electrodes corresponding to the respective fields, and is configured to display an image in response to data and scan signals. The data and scan drivers respectively supply the data and scan signals to the display unit. The power supply has a first output terminal for outputting a first power and a plurality of second output terminals for outputting a plurality of second powers to the plurality of cathode electrodes. The driving voltage calculator calculates the voltage of each of the second powers for a respective one of the cathode electrodes based on a magnitude of a respective one of the data signals.