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In this paper, we consider opportunistic spectrum sharing when the primary system experiences unfavorable channel conditions. In the proposed spectrum sharing protocol, the secondary system tries to help the primary system to achieve its target rate by acting as an amplify-and-forward relay and allocating a fraction of its subcarriers to forward the primary signal. As a reward, the secondary system uses the remaining subcarriers to transmit its own signal, and thus gaining spectrum access. We study the joint optimization of the set of subcarriers used for cooperation, subcarrier pairing and secondary subcarrier power allocation such that the transmission rate of the secondary system is maximized, while helping the primary system, as a higher priority, to achieve its target rate. Simulation results demonstrate that both primary and secondary systems benefit from the proposed opportunistic spectrum sharing protocol.