The recently proposed IEEE P802.3ba 100 Gb/s Ethernet (100 GbE) standard has adopted 100 Gb/s transmission over 10 and 40 km of single-mode fiber (SMF) using four-channel (4 × 25 Gb/s) wavelength-division-multiplexed (WDM) systems, which is neither cost-effective nor spectrally efficient compared with a single-channel system exploiting the combination of higher-order modulations and optical orthogonal frequency division multiplexing (O-OFDM). This paper demonstrates that a spectrally efficient (4 bits/s/Hz) single-channel 100 Gb/s system can be designed based on 64-quadrature amplitude modulation (64-QAM) and directly detected O-OFDM (DDO–OFDM) with an effective OFDM signal bandwidth of 24 GHz. Such a system can not only offer error-free (at bit error ratio of 10⁻³ without forward error correction) transmissions over the targeted maximum distance of 40 km of SMF but also achieves a power margin of 16 dB without any inline amplifier or dispersion compensation. This confirms that the proposed system has the potential to offer 100 Gb/s Ethernet for both point-to-point short communication links and 1:32 split passive optical networks (PONs). The bit rate of the system is then increased to 1 Tb/s employing WDM, and it is found to have equal potential for point-to-point short communication links and 1:16 split PON. Finally the reach limits of both of the proposed systems are quantified.