A data processing method and an apparatus for an organic light emitting diode (OLED) display device are provided. The method includes modulating input data using a maximum degradation compensation gain, compensating degradation of the modulated data using a degradation compensation gain, accumulating the degradation-compensated data, determining a degree of degradation of each of sub-pixels, based on the accumulated data, detecting a degradation compensation gain in accordance with the determined degradation degree, storing the detected degradation compensation gain, and outputting the stored degradation compensation gain, detecting a maximum one of degradation compensation gains of respective sub-pixels, and outputting the detected maximum degradation compensation gain, analyzing the input data, thereby setting a peak luminance control (PLC) gain, and modulating the PLC gain, using the output maximum degradation compensation gain, and analyzing the degradation-compensated data, thereby detecting a peak luminance, and adjusting the detected peak luminance, using the modulated PLC gain.