Considering frequency-selective Nakagami-m fading multiple-input multiple-output (MIMO) channels in the presence of spatial correlation, we study the error probability of orthogonal space-time block-coded (OSTBC) orthogonal frequency-division multiplexing (OFDM) systems. Exploiting the equivalent single-input single-output (SISO) OFDM model, we derive an exact and closed-form expression for the moment generating function (MGF) of effective instantaneous signal-to- noise ratio (SNR). By using a well-known MGF-based approach and the Gauss-Legendre quadrature rule, a simple and accurate numerical solution for the average symbol error rate (SER) of coherent MPSK signals is provided. In addition, the impact of complex component correlation among the MIMO channel links on the SER performance is assessed by fading figure and numerical examples.