In optical communication systems for single-carrier transmission based on orthogonal frequency-division multiplexing (OFDM), high power signals result in fiber nonlinear effects such as self-phase modulation (SPM), which causes inter-symbol interference (ISI) between sub-carriers. To reduce peak-to-average power ratio (PAPR) at a transmitter of the optical OFDM system, a partial transmit sequence (PTS) technique can be applied, but it requires reduced computational complexity for practical implementation because the complexity in an optimum search over all combination of phase factors is exponentially increased with the number of sub-blocks. In this paper, a PTS method with low computational complexity is proposed by modifying an artificial bee colony (ABC) algorithm, which is an approach based on solution sets to solve combinatorial and numerical optimization problems. The effectiveness of the proposed method is confirmed by simulation results of complementary cumulative distribution function (CCDF) and BER in a coherent optical OFDM (CO-OFDM) system.