The Oi Formation is a palaeontological formation located in the Ichishi region of Central Japan. It is part of the Miocene Ichishi Group, which dates to the Lower Miocene period. The Kamimitsugano tuffaceous sandstone members of the Oi Formation were found to contain two of the oldest fossils of still existent species (the Ophiomusium lymani and Ophiochiton cf. fastigatus).
- MATSUOKA, HIROSHIGE; SAKAKURA, FUJIO; OHE, FUJIO (December 30, 1998). "A Miocene Pseudodontorn". Paleontological Research. 2 (4): 246. Retrieved 23 December 2014.
- Feral, Jean-Pierre; Bruno, David (2001). Echinoderm Research 2001. CRC Press. p. 56.
- "Ophiomusium lymani". Encyclopedia of Life. National Museum of Natural History (USA). Retrieved 23 December 2014.