We propose the optimal resource allocation for wireless multi-destination relay systems with orthogonal frequency division multiplexing (OFDM) modulation, which includes subchannel allocation (SA), subchannel pairing (SP), and joint source-relay power allocation (PA). Our work is different in that we consider a practical downlink OFDM based cooperative network, where the relay is used to help the source (base station) to communicate with multiple destinations (users) rather than a single destination as assumed in the previous work. Simulation results demonstrate that the proposed optimal resource allocation outperforms other resource allocation schemes in terms of the system capacity. This work can be extended to multi-relay multi- destination uplink/downlink OFDM relay systems.