This paper describes the application of the microwave subcarrier multiplexing technique (SCM) in an optical frequency division multiplexing (OFDM) system which employs an optical FM direct detection scheme. Direct frequency modulation of a laser diode offers the advantages of compact optical spectrum and simple driving circuit; it also permits a more effective utilisation of optical channel capacity. The carrier-to-noise ratio performance of each subcarrier channel is evaluated. The effect of laser diode phase noise and relative intensity noise as well as the intermodulation product of subcarrier channels on the overall system performance is analysed and the results presented.