The performance assessment of orthogonal frequency division multiplexing (OFDM) signals in direct-detection (DD) transmission systems by using the error vector magnitude (EVM) and several bit error ratio (BER) approaches is analyzed and compared through numerical simulation. It is shown that an excellent accuracy of the BER estimates is obtained by a semi-analytical Gaussian approach for all the OFDM system configurations analyzed and that the EVM only provides reliable estimates of the system performance when the system is dominantly impaired by noise. Additionally, a novel Q-factor approach for OFDM optical signals showing improved BER estimates is also presented.