An efficient post-coding strategy is proposed in this letter to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals for optical intensity modulated direct detection (IM/DD) systems. The post-coding scheme based on discrete cosine transform (DCT) is employed after the inverse fast Fourier transform (IFFT) in the transmitter to reduce the PAPR of OFDM signals. This method is different from the conventional pre-coding scheme which is employed before IFFT operation. Numerical simulations demonstrate that the new DCT post-coding strategy can significantly reduce the PAPR than the conventional pre-coding scheme. Meantime, the bit error rate (BER) performance of the proposed post-coding system can be improved compared with the conventional pre-coding scheme.