Animals

Old World monkey

Old World monkeys (Cercopithecidae): diversity, behaviour and conservation

Old World vulture

Old World warbler

Oligochaete

Oligochaete — segmented worms of the class Clitellata

Olinguito

Olm

Olorotitan

Olorotitan — a crested hadrosaur from the Late Cretaceous

Omul

Onager

Onager (Equus hemionus): the Asian wild ass

Oncilla

Oncilla (Leopardus tigrinus): small spotted Neotropical wild cat

Opabinia

Opabinia — a strange Cambrian animal with five eyes

Ophthalmosauridae

Ophthalmosauridae: the large‑eyed ichthyosaur family

Ophthalmosaurus

Ophthalmosaurus — a large-eyed Jurassic ichthyosaur

Opiliones

Harvestmen (Opiliones): Biology, Diversity, Fossils, and Ecology

Opisthocoelicaudia

Opisthocoelicaudia: a Late Cretaceous Mongolian titanosaur

Opossum

Opossum (Didelphimorphia): biology, behavior, and ecological role

Orange-breasted sunbird

Orange-thighed frog

Orangutan

Orangutan (genus Pongo)

Oreophrynella

Oriental Longhair

Oriental magpie-robin

Oriental magpie-robin (Copsychus saularis)

Oriental turtle dove

Oriole

Oriole (Old World orioles, genus Oriolus)

Ornithischia

Ornithocheirus

Ornithocheirus: a crested pterosaur genus

Ornitholestes

Ornitholestes: a small Late Jurassic coelurosaur

Ornithomimidae

Ornithomimus

Ornithopod

Ornithopod dinosaurs: bird-hipped herbivores and their evolution

Ornithurae

Orthoptera

Orthoptera: grasshoppers, crickets and katydids — biology and importance

Oryx

Oryx: antelopes adapted to deserts and dry grasslands

Oscar (fish)

Oscar (Astronotus ocellatus) — popular South American cichlid

Osornophryne

Osprey

Osprey (Pandion haliaetus): the fish-eating raptor

Ostafrikasaurus

Ostafrikasaurus: Late Jurassic theropod known from a single tooth

Ostracod

Ostracoderm

Ostracoderms: Armoured Jawless Fishes of the Early Paleozoic

Ostreidae

Oswego bass

Otter

Ouranosaurus

Ouranosaurus: the sail-backed iguanodontid of Early Cretaceous Africa

Oviparous animals

Oviparous animals: egg-laying reproduction and its diversity

Oviparous animals reproduce by laying eggs. This article explains egg structure, fertilization modes, examples across animal groups, evolutionary significance, and differences from

Oviraptor

Oviraptor — toothless, crested theropod of the Late Cretaceous

Oviraptorosaur

Oviraptorosaurs: the beaked, feathered maniraptorans of the Cretaceous

Ovis

Ovis — genus of sheep (wild and domestic)

Owl

Owl (Strigiformes): biology, behavior and global distribution