In this letter, we derive a closed-form solution of the exact pairwise error probability (PEP) for Rayleigh fading orthogonal frequency-division multiplexing (OFDM) channels with multiple transmit and receive antennas. We begin with the case of frequency-selective quasi-static fading OFDM channel model. The analysis is then generalized to the frequency-selective block fading OFDM channel model. This is a suitable model for cooperative space-time coded OFDM systems, where, due to the spatial separation between the cooperating users, the signal-to-noise ratio (SNR), the number of antennas, and the number of nonzero channel taps during each user's transmission (hence, in each block) may be different. We provide numerical examples to illustrate the usefulness of the derived exact PEP expressions.