This work addresses theoretically some issues related to CO-OFDM system and identifies some parameters that affect its performance. The performance of a long-haul CO-OFDM system is simulated taking into account the effect of fiber chromatic dispersion, polarization mode dispersion, and nonlinear fiber optic effects. A theoretical study regarding the relation between the number of subcarriers and the mean launched optical power is carried out and supported by simulation results. The study reveals that there is an optimum launched power value for a fixed number of subcarriers. Further, The inphase/quadrature (I/Q) amplitude and phase imbalance effects are studied in CO-OFDM and an analytical model for the I/Q imbalance is developed and supported by simulation results. The results indicate that the I/Q imbalance degrades the BER performance considerably.Simulation results are obtained using OptiSystem (version 9.0) software package.