We investigated the effect of subcarriers-to-subcarriers mixing interference and frequency fading in direct-detection (DD) optical orthogonal frequency-division multiplexing (OFDM) system. We also proposed and experimentally demonstrated a novel DD OFDM transmission scheme with pre-emphasis technique. By pre-emphasizing the power of the OFDM subcarriers at the high frequency, the received sensitivity at our experiment was improved about 2 dB at a bit error rate of $1 x 10^-4 after 100-km SMF-28 transmission compared to the regular one without this technique.