Subcarrier signal-to-noise ratio (SNR) expressions are derived in this paper for the precoded direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system and the precoded asymmetrically clipped O-OFDM (ACO-OFDM) system. Simulation results show that in the line-of-sight channel, precoding does not affect the bit error rate (BER) performance of the systems. In the multipath channel, however, it can equalize the frequency-selective fading of the optical wireless channel, thereby uniformizing the subcarrier SNR at the receiving terminal of the O-OFDM system and ultimately improving the system BER performance. The Monte Carlo method is then used to simulate and analyze the peak-to-average power ratio (PAPR) and the BER of O-OFDM systems applying discrete Fourier transform precoding, discrete cosine transform precoding, and discrete Hartley transform precoding, which verifies the correctness of the theoretical analysis. The simulation also verifies the PAPR suppression function of precoding under bilateral clipping of O-OFDM signals.