The existing clipping researches in direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) systems generally originate from insufficient DC bias and the nonlinear transmission characteristics of physical devices which will distort the system performances. In contrast to conventional clipping theories, the beneficial clipping method demonstrated in this paper aims to improve the transmission effects of DCO-OFDM systems. Using the Bussgang theorem, the signal to noise ratio (SNR) and bit error ratio (BER) of DCO-OFDM systems with the beneficial clipping method are modeled mathematically. It is found that the beneficial clipping method can effectively reduce the system BER, compared with the no clipping situation, when the clipping ratio is mapped over an appropriate range. Also, the optimal clipping ratio changes with variation of the modulation depth. These results illustrate that the beneficial clipping method can enhance the performance of DCO-OFDM systems, although it does introduce clipping noise.