In this paper we propose an interleaved spread spectrum orthogonal frequency division multiplexing (ISS-OFDM) system for achieving both frequency diversity and time diversity and investigate the system performance over frequency selective fading channels. The purpose is to exploit the diversity capability of OFDM systems, develop efficienct spectrum spreading technique and improve the system performance against frequency selective channel fading. At the transmitter, the ISS-OFDM signal is generated by employing spread spectrum modulation and interleaving techniques. At the receiver, the received signals are combined by using maximum ratio combining (MRC) technique. The simulation indicates that the ISS-OFDM system has improved performance in multipath fading channels. Another unique characteristic is that the spectrum spreading factor and diversity order provided by the system are reconfigurable to achieve cognitive communications.