O. rex
# O. rex

O. rex may refer to:

* Oecomys rex, the regal oecomys or king arboreal rice rat, a rodent species found in Guyana, Suriname, French Guiana and nearby parts of Venezuela and Brazil
* Ogasawarana rex, a land snail species endemic to Japan
* Othnielia rex, a hypsilophodont dinosaur species
