As a key module in the orthogonal frequency division multiplexing (OFDM) system, the synchronization module is the basic premise for data demodulation and channel estimation. Due to the sensitivity of OFDM-based visible light communication (VLC) systems to synchronization errors, the accuracy of symbol timing offset (STO) estimation directly affects the system performance. An STO estimation method based on odd-even symmetry is presented for direct current biased optical OFDM (DCO-OFDM) systems. By designing training symbols with an odd-even symmetric structure, this method can generate an ideal pulse-like timing metric, thereby achieving excellent STO estimation accuracy. The performance of the proposed method is assessed by measuring the root-mean-square error (RMSE), mean absolute error (MAE), and the bit error rate (BER) of timing offset estimation through simulation. It is then compared with the performance of two baseline methods (Park method and Guerra method). The simulation results show that this novel symbol timing synchronization approach is superior to the above methods in the additive white Gaussian noise (AWGN) channel and the multipath fading channel, which verifies the effectiveness of this method in DCO-OFDM systems.