Coherent optical orthogonal frequency-division multiplexing (COOFDM) is being considered as a promising technology for future optical transmission. In COOFDM systems, optical Mach–Zehnder modulators (MZM) are used to up-convert orthogonal frequency-division multiplexing (OFDM) signal from radio frequency (RF) domain to optical domain. However, OFDM is very sensitive to nonlinearity, so subsequently study of nonlinearity impact from MZM is of great interest. The impact of COOFDM systems introduced by the nonlinearity of MZM is focused on and a numerical simulation to identify system Q-value under various MZM bias points and modulation index is performed. Results show that in contrast to intensity-modulation/direct-detected system (bias point is quadrature point), optimal modulator bias point is zero point and optimal modulation index is 0 dB for COOFDM system.