The Ohyamashimo Formation(大山下層) is an Early Cretaceous (Albian) geologic formation in Japan.

It has been dated to the early-mid Albian, between 112.1 ± 0.4 Ma and 106.4 ± 0.4 Ma.

Dinosaur remains have been discovered from this formation, including the sauropod Tambatitanis, the theropod Hypnovenator and the ceratopsian Sasayamagnomus.

Other representative fossils from the formation include the monstersaurian lizard Morohasaurus and eutherian mammal Sasayamamylos.

The depositional environment represents a fluvial system with a subhumid to semi-arid climate.