A power system for an organic light emitting diode (OLED) display includes a power supplier and a power source controller. The power supplier respectively supplies a first power source voltage and a second power source voltage to first and second power source voltage application lines. The power source controller calculates a reference power source voltage corresponding to a maximum average grayscale using a distribution for each grayscale of first to third image data, models each voltage drop of the first and second power source voltages for first to third subpixels, and reflects the voltage drop to the reference power source voltage to change the second power source voltage.