Disclosed is an organic light emitting display and a driving method thereof capable of reducing the number of output lines in a data driver. A scan driver sequentially supplies a scan signal to a scan line during a second period of one horizontal period. A data driver includes a plurality of output lines, for supplying a plurality of data signals to the respective output lines during the second period. Demultiplexers are installed at the respective output lines, and include a plurality of data transistors for supplying the data signals to the output lines during the second period, to a plurality of data lines. Initializing sections are installed between a first initialization power supply and the plurality of data lines, and include a plurality of initialization transistors for supplying a voltage of the first initialization power supply to the plurality of data lines. A pixel portion includes a plurality of pixels positioned at areas partitioned by the scan line and the data lines, and the initialization transistors are turned-on during a first period of the one horizontal period, which is not overlapped with the second period.