We proposed a novel flip-optical orthogonal frequency division multiplexing (OFDM) scheme based on fast Hartley transform (FHT), namely Flip-FHT. FHT, as a real trigonometric transform, was used to process the Fourier transform. Therefore, Hermitian symmetry was not required for the input signal. At the same time, unlike the traditional fast Fourier transform (FFT)-based asymmetric clipped optical OFDM (ACO-OFDM), in which only odd subcarriers carried information symbols, the proposed scheme flipped the negative part of the real bipolar signal so that even subcarriers could also carry information symbols. Therefore, this scheme is suitable for intensity modulation/direct detection (IM/DD) optical wireless communication (OWC) systems. The spectral efficiency of the Flip-FHT system is 2 times that of the ACO-OFDM system, and the computational complexity at the receiver is 50% that of the ACO-OFDM system. Simulation results show that compared with the ACO-OFDM system under 4-quadrature amplitude modulation (4QAM) and 16QAM, the Flip-FHT system featuring binary phase-shift keying (BPSK) and 4-pulse amplitude modulation (4PAM) achieves the same performance with a simpler system and a smaller constellation size.