A novel multi-core fiber (MCF), having eight cores within a 125-μm-diameter cladding and suitable for short-reach transmissions, is proposed. By optimizing O-band transmission, the proposed MCF realized the high-density core packing simultaneously achieving the optical characteristics comparable to those of the standard single-mode fiber in O-band, where low material dispersion can be leveraged. The standard 125-μm-diameter cladding is compatible with conventional cabling/connecting technologies and has the tightly bendable mechanical reliability proven by the widely installed standard optical fibers. An ultra-high-density 12-MCF cable was fabricated with the proposed MCF, and achieved 13.6 cores/mm 2 (96 cores in the 3-mm-diameter cable). The transmission characteristics of the cabled MCF were evaluated using a 100GBASE-LR4 transceiver, and error-free 8-SDM × 4-WDM × 25-Gb/s (800-Gb/s/fiber) signals in O-band were successfully transmitted over the 1.11-km-long cabled MCF.