Hello,
You may transmit OFDM signal over the FSO channel in OptiSystem. You may use the Noise Source component to add noise to control the SNR or Set OSNR component to control the OSNR in your system.
There are few examples on OFDM in OptiSystem Example library located at the following link that can be used for guidance.
C:\Users\USER NAME\Documents\OptiSystem 20.0 Samples\Advanced modulation systems\OFDM systems
regards,
Ahmad