The invention relates to the field of optical transmission systems. In particular, the invention relates to a system and a method for adjusting an optical OFDM transmission system in a power optimized manner. An optical OFDM transmitter (310) operating at an overall bit-rate is provided. It comprises an adjustable mapping unit (314) associated with one of N OFDM subcarriers, operable to map M bits of a digital input signal (360) into a constellation point, thereby yielding a subcarrier signal of the corresponding OFDM subcarrier. Furthermore, it comprises an adjustable transformation unit (315, 316), operable to transform the subcarrier signal to yield an electrical output signal. In addition, the OFDM transmitter comprises an electrical-to-optical converter (324, 325), operable to convert the electrical output signal into an optical output signal. Finally, the OFDM transmitter comprises a control unit (311, 312), operable to select the number of OFDM subcarriers N and the number of bits M, such that the electrical power consumption of the OFDM transmitter (310) is minimized for the overall bit-rate and operable to adjust the adjustable mapping unit (314) and the adjustable transformation unit (315, 316) according to the selected numbers M and N.