In this paper, we investigate orthogonal frequency division multiplexing (OFDM) technique for distributed spacetime block coding (D-STBC) in a relay-assisted transmission scenario over frequency-selective fading channels. We consider the special case of a single-relay where the source-to-relay (S → R), relay-to-destination (R → D) and source-to-destination (S → D) links experience possibly different channel delay spreads. Our analysis demonstrates that uncoded distributed OFDM-STBC (D-OFDM-STBC) scheme achieves a maximum diversity order of two for the considered single-relay scenario since it is able to exploit only the spatial diversity, but not the available rich multipath diversity. We further consider a combination of D-OFDM-STBC and Trellis Coded Modulation (TCM) with frequency-interleaving. Under the assumption of perfect power control for the relay terminal and high signal-to-noise ratio for the underlying links, our performance analysis demonstrates that D-OFDM-STBC scheme with judiciously designed outer TCM codes, i.e. sufficiently large effective code length (ECL), is able to achieve a maximum diversity order of min(L1, L3) + L2 + 2 where L1, L2, and L3 are the channel memory lengths for S → R, S → D and R → D links, respectively. This illustrates that the smaller of the multipath diversity orders experienced in S → R and R → D links becomes the performance bottleneck for the relaying path. An extensive Monte Carlo simulation study is presented to corroborate the analytical results and to provide further insights into the performance.