Joint power allocation, relay selection, and sub-carrier assignment are critical and challenging for achieving full benefits of OFDM based cooperative relay networks. In this paper, we study such a problem in a dual-hop multi-relay OFDM system with an objective of maximizing the spectral efficiency under a total power constraint. The system consists of a pair of source and destination and multiple decode-and-forward relays. We formulate the joint optimization of the three types of resources: power, subcarrier and relay nodes, as a problem of subcarrier-relay assignment and power allocation. We show that it can be decomposed into 2N + 1 sub-problems through dual relaxation, with N being the total number of subcarriers. An optimal algorithm with polynomial complexity is presented. A suboptimal algorithm that decouples the subcarrier-relay assignment and power allocation is also proposed to tradeoff between performance and computational complexity.