Fiber nonlinearity of optical orthogonal frequency division multiplexed (OFDM) system restricts the capacity improvement of optical fiber transmission. In this paper, we propose a novel digital coherent superposition (DCS) scheme to improve the tolerance to fiber nonlinearity in a coherent optical orthogonal frequency division multiplexed system. In simulation, 71.53 Gbit/s orthogonal frequency division multiplexed signal per channel with Hermitian symmetry is transmitted over 400 km standard single mode fiber in a wave division multiplexed-polarization-division multiplexed-coherent optical orthogonal frequency division multiplexed system with five channels. The 4-quadrature amplitude modulation is used for symbol mapping. For the receiver, after the conventional OFDM signal processing, we conduct DCS for OFDM subcarrier pairs, which requires only conjugation and summation in the x, y polarization direction, respectively. Firstly, the channel spacing is 25 GHz, the maximum signal-to-noise ratio improvement is 9.05 dB or 6.02 dB with or without symmetric dispersion compensation compared with a conventional orthogonal frequency division multiplexed system. The optimum launch power is increased by 2 dB. Secondly, the channel spacing is changed to 50 GHz to investigate the nonlinearity tolerance at different channel spacings in the wave division multiplexed system, the maximum signal-to-noise ratio improvement is 8.75 dB or 4.9 dB with or without symmetric dispersion compensation, respectively. Theoretical and simulation analysis show that the proposed method in this paper can effectively mitigate the first-order nonlinear distortions and hence improve the tolerance of coherent optical orthogonal frequency division multiplexed system with different channel spacings to fiber nonlinear effects.