In this paper, we study a decode-and-forward cooperative diversity scheme for wireless systems using the combination of bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). We propose a simple cooperative maximum-ratio combining scheme for the destination which can successfully exploit the full spatial and frequency diversity offered by the channel. Taking possible decision errors at the relay into account, we develop a closed- form upper bound on the asymptotic worst-case pairwise error probability (PEP) of the considered cooperative diversity system which provides insight into the influence of various code and channel parameters on performance. The PEP expression is then exploited for optimal relay placement. Simulation results corroborate the derived analytical results regarding the diversity gain of the system and confirm the effectiveness of the proposed relay placement scheme.