In this paper, we consider a cooperative multi-carrier multi-relay system over frequency-selective fading channels. Specifically, we adopt the orthogonal frequency division multiplexing (OFDM) with bit-interleaved coded-modulation (BICM) as the physical layer signalling platform. Under the assumption of amplify-and-forward relaying, we first present a diversity order analysis through the derivation of pairwise error probability (PEP) for an orthogonal scheme in which relays scale and transmit their received OFDM signals in non-overlapping time slots. Based on the derived PEP expression, we further propose a relay selection rule to improve the spectral efficiency. We also investigate some suboptimal (heuristic) selection methods as low-complexity alternatives. We present extensive Mont Carlo simulations to confirm the analytical derivations and provide performance comparisons among competing schemes.