This paper presents a performance enhancement for a 40 Gbit/s intensity-modulated direct-detected (IM/DD) optical orthogonal frequency-division multiplexing (OFDM) system, focusing on minimizing the peak to average power ratio (PAPR) using a selective mapping (SLM) scheme. The analysis evaluates key performance metrics, including launched power, optical signal to noise ratio (OSNR), propagation length, power spectral density (PSD), and bit error rate (BER). The implementation of the SLM technique significantly reduces the PAPR from 10.4 dB to 5.55 dB at a complementary cumulative distribution function (CCDF) of 10-3, achieving a 4.85 dB reduction. These results demonstrate the effectiveness of SLM in mitigating high PAPR in our system, which can improve tolerance to system nonlinearities while maintaining manageable power efficiency. Furthermore, the analysis suggests that additional PAPR improvements are achievable by increasing the number of SLM partition blocks. The SLM method outperforms the partial transmit sequence (PTS) in terms of PAPR reduction, PSD, and BER.