An organic light emitting display includes a display unit, a scan line, a data line, a reference line and a main voltage line. The display unit includes a first PMOS transistor, an OLED and a capacitor. The first PMOS transistor generates a driving current. The OLED emits light according to the driving current. The capacitor has a first end and a second end, and the first end and the second end selectively and respectively receives a data signal and a reference voltage according to a control signal. The scan line is for transmitting a scan signal. The data line is for transmitting the data signal. The reference line is for outputting the reference voltage. The main voltage line is for outputting a main voltage to the first PMOS transistor.