We consider a cognitive OFDM-based spectrum pooling wireless system where a source node desires to transmit data to a distant destination node with the assistance of a relay node. Communication between any transmitter-receiver pair occurs on a pool of OFDM subcarriers corresponding to spectrum found to be empty of primary users. In such a system, the source node uses a certain pool of subcarrier frequencies to transmit data to the relay. The relay then reassigns and forwards the received data on a possibly different pool of OFDM subcarriers, which is then decoded at the destination node. Our goal is to maximize the capacity of this spectrum pooling relay system. This is done by determining an optimum assignment of the subcarrier pools available between the source-relay and relay-destination.