Due to high peak-to-average power ratio (PAPR) and the nearest subcarrier spacing, fiber nonlinearity impairment becomes the determining factor of coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems. Midlink optical phase conjugation (M-OPC) is used to compensate for the fiber Kerr nonlinearity effect in CO-OFDM systems. Because of the symmetry of both ends of the link, OPC can best meet the conditions of compensation and has best nonlinearity compensation effect. And OPC can be applied to link with and without online dispersion compensation. Through OPC for 40 Gb/s single channel system, the maximum Q can be increased by 3 dB, and the nonlinear threshold (NLT) (for Q>10 dB) is increased by 4 dB. For wavelength division multiplex (WDM) system, the maximum Q can be increased by 3 dB and NLT is increased by 1 dB.