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We show that cross-phase modulation in coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is mitigated with a simple intensity-dependent phase-modulation before the wavelength demultiplexer. A single low-bandwidth photodiode is used to estimate the optical power of all wavelength-division-multiplexing channels entering the receiver, and this determines a phase modulation applied to all channels. This method works well in conjunction with self-phase-modulation pre/postcompensation, allowing significant improvements in the performance of OFDM systems over optically dispersion-managed links. Using simulations, we show a 4-dB signal quality improvement in a 2375-km 60-Gb/s CO-OFDM system in the nonlinear limit.