Coherent optical orthogonal-frequency-division-multiplexing (CO-OFDM) has drawn more attention in optical transmissions as an attractive modulation format for the forthcoming 100 Gb/s Ethernet. However, CO-OFDM system requires high-speed digital-to-analog converters (DAC) and analog-to-digital converters (ADC), which may not be available today. To resolve ADC/DAC bandwidth bottleneck, with the help of orthogonal-band-multiplexing (OBM) and polarization division multiplexing (PDM), 100 Gb/s CO-OFDM system based on OBM is presented. With this scheme, simulation is done to validate the feasibility of the system model and algorithm. The result shows that the performance of MIMO CO-OFDM system based on OBM is maintained above 13 dB at 0 GHz channel spacing for 800 km standard single mode fiber (SSMF) transmission without any inline dispersion compensation and polarization controller (PC) and the DAC/ADC do not need to operate at extremely high sampling rate.