Dear Hussien,
you may try the examples in OptiSystem Example library on OFDM and PON for guidance (links are below). You need to create your own Rough ONU base on your overall system design.
C:\Users\USER NAME\Documents\OptiSystem 18.0 Samples\Metro and access systems\PON and CDMA systems
C:\Users\USER NAME\Documents\OptiSystem 18.0 Samples\Advanced modulation systems\OFDM systems
Regards,
Ahmad