The hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) technique only utilizes the odd subcarriers and the imaginary part of the even subcarriers for symbol transmission, leading to the problem of low spectral efficiency. To address this problem, an adaptively biased HACO-OFDM (AB-HACO-OFDM) method is proposed. In the proposed method, a pulse-amplitude modulated discrete multitone (PAM-DMT) signal is designed for the unused real part of the even subcarriers, and is further superimposed on the HACO-OFDM signal for hybrid transmission to enhance the subcarrier utilization. Meanwhile, a power-efficient adaptive bias signal is introduced to guarantee the non-negativity. The simulation results demonstrate that compared to HACO-OFDM, the proposed AB-HACO-OFDM is capable of effectively improving the spectral efficiency of the transmission, while at the same time, achieving better performance in bit-error rate (BER). Additionally, compared to the conventional orthogonal frequency division multiplexing (OFDM) methods, the proposed method requires much lower signal-to-noise ratio to achieve the BER of target, thus exhibiting higher power efficiency.