Hello,
The example (FSO_Terrestrial.osd) located at C:\Users\USERNAME\Documents\OptiSystem 15.2 Samples\Optical wireless\Terrestrial link has the parameters for FSO channel used. Regarding the OFDM, you may use the examples at C:\Users\USERNAME\Documents\OptiSystem 15.2 Samples\Advanced modulation systems\OFDM systems to get access to the OFDM component settings.
I hope these links help you design your OFDM FSO project.
regards,
Ahmad