In this paper, we suggest to optimize power allocation coefficients and harvesting duration in mixed Radio Frequency (RF)/Free Space Optical (FS0) communications using Non Orthogonal Multiple Access (NOMA). The system model contains three time slots. In the first time slot, relay node R harvests energy from RF signal received from node H. Node H can be any node transmitting RF signals. In the second time slot, an FSO transmitter T transmits a linear combination of symbols of K users to relay node R using NOMA. In the third and last slot, relay R detects the receive symbols and sends a linear combination of detected symbols to K users using NOMA. We show that power allocation and harvesting duration optimization allow to increase the throughput of RF/FSO communications. We consider two users’ ranking techniques using average or instantaneous channel gains.