We introduce an exact theoretical approach for computing the symbol error rate of a M-QAM orthogonal frequency division-multiplexing (OFDM) system in the Cartesian clipping channel. Cartesian clipper clips the discrete time domain samples of the I/Q (in-phase/quadrature) components of the complex baseband OFDM signal separately. An exact expression is derived for the probability density function (PDF) of the clipping noise at the output of OFDM demodulator. Using this PDF, we can compute the performance of the OFDM system for each subcarrier. This approach has the accuracy of the simulation approach while it is not time consuming.