In this paper, we consider an orthogonal Multiple Access Multiple Relay Network (MAMRN), where several nodes cooperate to send their messages to a single destination. Frequency Division Multiplexing (FDM) is adopted for the sources and the relays, and the destination acts as the central node responsible for scheduling the cooperative retransmissions. Since FDM is used for orthogonality, different nodes are allocated at each sub-band of the transmission and retransmission phases. In this work, we present the system model of the considered orthogonal MAMRN when the FDM mechanism is adopted, while including the analytical derivations of the utility metrics (spectral efficiency and outage events). Then, two centralized node selection strategies are proposed. Moreover, we present the control information exchange process between the destination and the different nodes. The proposed strategies allocate for each sub-band the node that will transmit (or retransmit) with the goal of maximizing the spectral efficiency. Our numerical analysis considers both symmetric and asymmetric channel and source rate scenarios, and shows that the proposed algorithms outperform the algorithms used in the prior-art, and achieve a spectral efficiency that is close to the upper bound calculated by an exhaustive search approach while reducing the complexity and the overhead.