Abstract In the paper, a simple generation scheme of the twin-SSB-OFDM signal based on one OFDM transmitter is proposed in the direct-detection optical orthogonal frequency-division multiplexing (DDO-OFDM) system. Compared with traditional generation scheme that requires two OFDM transmitters, it has lower implementation complexity and lower cost. In addition, to enhance the spectral efficiency (SE) for DDO-OFDM system, an efficient bit-interleaved polar coded modulation (BIPCM) scheme adopting 256 quadrature-amplitude modulation (256-QAM) is proposed. Moreover, a quadratic polynomial permutation (QPP) interleaver is adopted for polar coded modulation (PCM) to further improve the overall system performance. The simulation results exhibit that the proposed scheme can achieve more than 4 dB and 5 dB Q-factor improvement compared to the conventional schemes with PCM and without BIPCM, respectively. At the same time, aided by the efficient 256-QAM BIPCM scheme, the receiver sensitivity is improved and the laser linewidth requirement is less strict compared to the conventional schemes.