In this paper we consider an adaptive modulation system with multiple-input multiple-output (MIMO) antennas in conjunction with orthogonal frequency division multiplexing (OFDM) operating over frequency selective Rayleigh fading environments. In particular, we consider a type of beamforming with a maximum ratio transmission, maximum ratio combining (MRT-MRC) transceiver structure. For this system we derive a central limit theorem for various block-based performance metrics. This motivates an accurate Gaussian approximation to the system data rate and the number of outages per OFDM block. In addition to data rate and outage distributions, we also consider the subcarrier SNR as a random process in the frequency domain and compute level crossing rates (LCRs) and average fade bandwidths (AFBs). Hence, we provide fundamental but novel results for the MIMO OFDM channel. The accuracy of these results is verified by Monte Carlo simulations, and applications to both performance analysis and system design are discussed.