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.[1] 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).[2][3]