In this paper, we investigate resource allocation algorithms for Orthogonal Frequency Division Multiplexing (OFDM) based dual hop systems including a source, multiple relays, and multiple destinations and utilizing Decode-and-Forwarded (DF) relaying. The main objective is to maximize the spectral efficiency under the total power constraint. We first propose an algorithm which simultaneously allocates transmission power and transmission time used by each hop in every time frame. Then we propose a more tractable algorithm which decomposes the original problem in two subproblems. The first subproblem is to solve the optimization problem for a given value of transmission time used by each hop, and the second subproblem is to compute the value of transmission time. The discussed algorithms are also compared with each other and with other resource allocation schemes. Simulation results are also provided which verify the performance improvement by adopting of the proposed algorithms.