Extending the reach of traditional passive optical networks (PONs) to 100 km and increasing the split ratio beyond 1:64 are promising solutions in future optical access networks. These systems can accommodate increased users at longer distances potentially at low cost. With the increasing demand for higher bandwidths, current networks may soon require that bit rates upgrade to 100 Gb/s and beyond. However, the traditional on-off-keyed PON cannot be scaled up to such bit rates, as very high-speed opto-electronic devices are required that are still maturing. Therefore, to provide a comprehensive solution to these scalability issues of existing PONs, we propose a spectrally efficient (4 bit/s/Hz) 100 Gb/s long-reach PON based on 64 quadrature amplitude modulation (QAM) and frequency interleaved directly detected optical orthogonal-frequency-division multiplexing. We show that the proposed system may operate effectively over 100 km of single mode fiber with a 1024-way split and a receiver bandwidth of 25 GHz. It is also shown that the system can be provisioned to support even higher numbers of users (e.g., 2048, 4096, etc.) simply by varying the order of QAM with little compromise in bit rates. Moreover, the effects of various link parameters such as laser linewidths, fiber dispersion, filter profiles, etc. are also investigated for proper link dimensioning.