A simplified expression allowing the estimation of the error vector magnitude (EVM) of each subcarrier of orthogonal frequency-division multiplexing (OFDM) direct-detection wavelength division multiplexing systems dominantly impaired by the cross-phase modulation is proposed and validated. This expression is then extensively used to assess the dependence of the OFDM signals performance on the number of optical channels transmitted, on the average optical power per channel at the fibre input, and to evaluate the capacity of OFDM-based quintuple play service provisioning in long-reach passive optical networks.