In this study, the authors analyse the performance of the free space optical (FSO) transmission link using orthogonal frequency division multiplexing (OFDM) modulation with M-ary quadrature amplitude modulation (QAM) encoder. For strong and weak atmospheric attenuation, the analysis of channel distribution in FSO is carried out using gamma-gamma model. In this analysis, the expressions for probability density function and cumulative density function of OFDM-based FSO channel have been derived in terms of Meijer's G-function. Also, the bit error rate and outage probability of M-ary QAM-based OFDM for an FSO transmission link has been derived. The feasibility of the system and the derived expressions are verified using Opti-system and MATLAB. From the analysis, it is observed that the performance of OFDM using M-ary QAM modulation is feasible for 7000 m, owning to its simplicity and good performance. © The Institution of Engineering and Technology 2016.