OFDM is playing a more and more important role in wireless communication systems. Unfortunately, it is not trivial to conduct a performance analysis of OFDM systems. Therefore, it is highly desired to develop an analytical design and performance analysis framework for OFDM channels. In this paper, we consider a unified performance metric for OFDM channels, which we shall refer to as outage exponent. The outage exponent, which is a special exponentially tight upper bound on outage probabilities, presents the fundamental relationship among the outage probability, target transmission rate, capacity of AWGN channel, SNR, and the number of diversity branches. The SNR gains of different coding schemes and the (finite-SNR and asymptotic) diversity-multiplexing tradeoff can be obtained from the outage exponent directly. In order to calculate the outage exponent for OFDM channels, we shall apply the large deviations theory, which will not only obtain an accurate estimation of the rate function, but also the coefficient of the exponential function. It is shown that the obtained outage exponent can allow the accurate estimation of the additional power required to decrease the outage probability by a specified value. Therefore, the outage exponent can be easily used to design and evaluate the performance of existing and upcoming OFDM systems.