In this paper, a hybrid technique using fast Hadamard transformation cascading a selective mapping (FHT-SLM) is proposed to reduce peak-to-average power ratio (PAPR) and complexity in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. The Hadamard transform is used to reduce the autocorrelation of signals so as to reduce the PAPR of signals. The fast Hadamard transform (FHT) algorithm is obtained by recursive decomposition of sparse matrix of a simple Hadamard matrix, which improves the computational efficiency and greatly reduces the computational complexity. Then, a selective mapping (SLM) scheme is used to select the smallest frame of PAPR in the time domain to further reduce PAPR of OFDM system. The simulation results show that at the complementary cumulative distribution function (CCDF) of 10^-4 , the PAPR of the proposed scheme FHT-SLM (V=8) is optimized to 7.63 dB. Meanwhile, when the bit error rate (BER) is 10^{-3, the optical signals to noise ratio (OSNR) of FHT-SLM (V=8) algorithm is 21.2 dB. The total computational cost of the FHT-SLM (V=8) is 28 672. Consequently, the simulation results prove that the proposed algorithm provides a choice for a balance among PAPR, BER and complexity.}