In this paper, we investigate a cooperative system with multiple relays and amplify-and-forward relaying over frequency-selective channels. To extract the available rich multipath diversity, we employ orthogonal frequency division multiplexing (OFDM) with precoding. Through the derivation of pair wise error probability (PEP), we demonstrate that the maximum diversity order is given by the summation of the channel length in the direct link and the minimum of channel lengths in each relaying link which is further confirmed by simulation results. Based on the PEPexpression, we also propose two relay selection strategies; one is on per-subcarrier basis and the other is on all-subcarriers basis. Our simulation results indicate that both strategies result in performance improvements.