I work on optical OFDM systems as a part of my university project and to compare the performance of CO-OFDM systems, I used three graphs in one of my reports:
1-BER vs. Transmission distance for different launch powers.
2-BER vs. OSNR for different transmission distances.
3-BER vs. Launch power of different Transmission distances.
Of course you can mix them in many ways, but I think these three graphs give you a good insight.