The problem of resource allocation is studied for a two-user fading orthogonal multiaccess relay channel (MARC) where both users (sources) communicate with a destination in the presence of a relay. A half-duplex relay is considered that transmits on a channel orthogonal to that used by the sources. The instantaneous fading state between every transmit-receive pair in this network is assumed to be known at both the transmitter and receiver. Under an average power constraint at each source and the relay, the sum-rate for the achievable strategy of decode-and-forward (DF) is maximized over all power allocations (policies) at the sources and relay. It is shown that the sum-rate maximizing policy exploits the multiuser fading diversity to reveal the optimality of opportunistic channel use by each user. A geometric interpretation of the optimal power policy is also presented.