 | Oak & Beech family (Fagaceae) |
| Oak, Cork (Quercus suber) |
| Oak, Silky (Grevillea robusta) |
| obesus, Sauromalus (see Sauromalus ater) |
| oblonga, Chelodina (Narrow-breasted Snake-necked Turtle) |
| obscurus, Chelidonichthys (Top Gurnard) |
| obsoletus, Pantherophis (Common American Rat Snake) |
| obsoletus, Pantherophis obsoletus (Black Rat Snake) |
| occidentalis, Leptopelis (Tai Forest Tree Frog) |
| occidentalis, Nephrurus levis (Smooth Knob-tailed Gecko) |
| occidentalis, Pulsatilla (Western Pasque Flower) |
| occipitalis, Tiliqua (Western Blue-tongued Skink) |
| occitanus, Buthus (Common Yellow Scorpion) |
| Oceanic Gecko (Gehyra oceanica) |
| Oceanic Snake-eyed Skink (Cryptoblepharus poecilopleurus) |
| oceanica, Gehyra (Oceanic Gecko) |
| ocellaris, Amphiprion (False Clown Anemonefish) |
| Ocellated Lizard, European (Timon lepidus) |
| Ocellated Snow Skink (Niveoscincus ocellatus) |
| ocellatus, Niveoscincus (Ocellated Snow Skink) |
| ochruros, Phoenicurus (Black Redstart) |
| Octopods, Squids, etc. (Cephalopoda) |
| Octopus Tree family (Didiereaceae) |
| Octopus vulgaris (Common Octopus) |
| Octopus, Common (Octopus vulgaris) |
| oculatus, Anolis (Dominican Anole) |
| oculatus, Lepisosteus (Spotted Gar) |
| Odd-toed hoofed mammals (Perissodactyla) |
| Odocoileus hemionus (Mule Deer) |
| Odonata (Dragonflies and Damselflies) |
| odoratus, Sternotherus (Common Musk Turtle) |
| Odorrana hosii (Hose's Rock Frog) |
| Oecophylla smaragdina (Green ant) |
| Oedura castelnaui (Northern Velvet Gecko) |
| Oedura marmorata (Marbled Velvet Gecko) |
| Oenpelli Rock Python (Morelia oenpelliensis) |
| oenpelliensis, Morelia (Oenpelli Rock Python) |
| officinalis, Rosmarinus (Rosemary) |
| Oil Beetle, Red-striped (Meloe majalis) |
| okinavensis, Ovophis (Okinawan Habu) |
| Okinawan Habu (Ovophis okinavensis) |
| Old Lady Cactus (Mammillaria hahniana) |
| Old Wife (Enoplosus armatus) |
| Old World Pitcher Plant family (Nepenthaceae) |
| Olea europaea (Common Olive) |
| Oleaceae (Olive family) |
| Oleander (Nerium oleander) |
| oleander, Nerium (Oleander) |
| Oleander, Yellow (Thevetia sp.) |
| oleracea, Eurydema (Cabbage Bug) |
| olivacea, Dasia (Olive Tree Skink) |
| olivaceus, Liasis (Olive Python) |
| olivaceus, Varanus (Gray's Monitor) |
| Olive family (Oleaceae) |
| Olive Python (Liasis olivaceus) |
| Olive Python, Papuan (Apodora papuana) |
| Olive Sea Snake (Aipysurus laevis) |
| Olive Tree Skink (Dasia olivacea) |
| Olive, Common (Olea europaea) |
| Ollotis alvaria (Sonoran Desert Toad) |
| Ollotis coccifer (Southern Roundgland Toad) |
| Ollotis conifera (Evergreen Toad) |
| Ololygon elaeochroa (see Scinax elaeochrous) |
| olor, Cygnus (Mute Swan) |
| Onagraceae (Evening-primrose family) |
| One-horned Chameleon, Giant (Chamaeleo melleri) |
| Oophaga histrionica (Harlequin Poison Frog) |
| Oophaga lehmanni (Red-banded Poison Frog) |
| Oophaga pumilio (Strawberry Poison Frog) |
| Open-litter Rainbow Skink (Carlia pectoralis) |
| Opheodrys aestivus (Rough Green Snake) |
| Ophidia (see Serpentes) |
| Ophiomorus latastii (Striped Snake Skink) |
| Ophiomorus punctatissimus (Greek Snake Skink) |
| Ophiophagus hannah (King Cobra) |
| Ophisaurus apodus (European Glass Lizard) |
| Ophisaurus apodus thracius (European Glass Lizard) |
| Ophisops elegans (European Snake-eyed Lizard) |
| Ophrys insectifera (Fly Orchid) |
| Opiliones (Harvestmen or daddy longlegs) |
| Opisthodon spenceri (Spencer's Burrowing Frog) |
| Oplurus cuvieri (Cuvier's Madagascar Spiny Iguana) |
| Orange (Citrus sinensis) |
| Orange-sided Bar-lipped Skink (Glaphyromorphus douglasi) |
| Orange-sided Forest Skink (Eulamprus amplus) |
| Orangespine Unicornfish (Naso lituratus) |
| Orange-spotted Day Gecko (Phelsuma guimbeaui) |
| Orangutan, Bornean (Pongo pygmaeus) |
| orbicularis, Emys (European Pond Turtle) |
| Orbweaver, Golden (Nephila madagascariensis) |
| Orchid (Bifrenaria tyrianthina) |
| Orchid (Guarianthe guatemalensis) |
| Orchid (Mormolyca ringens) |
| Orchid (Polystachya pubescens) |
| Orchid (Robiquetia sp.) |
| Orchid (Schomburgkia brysiana) |
| Orchid (Schomburgkia sp. (1)) |
| Orchid (Schomburgkia sp. (2)) |
| Orchid family (Orchidaceae) |
| Orchid Mantis, Malayan (Hymenopus coronatus) |
| Orchid, Baltic Marsh (Dactylorhiza majalis baltica) |
| Orchid, Christmas Star (Angraecum sesquipedale) |
| Orchid, Early Purple (Orchis mascula) |
| Orchid, Fly (Ophrys insectifera) |
| Orchid, Green-winged (Anacamptis morio) |
| Orchid, Pigeon (Dendrobium crumenatum) |
| Orchid, Rothschild’s Slipper (Paphiopedilum rothschildianum) |
| Orchid, Slipper (Paphiopedilum hookerae var. volonteanum) |
| Orchid, Slipper (Paphiopedilum wardii) |
| Orchid, Tongue (Serapias lingua) |
| Orchid, Western Marsh (Dactylorhiza majalis) |
| Orchidaceae (Orchid family) |
| Orchis mascula (Early Purple Orchid) |
| Oreamnos americanus (Mountain Goat) |
| Orectolobiformes (Carpet sharks) |
| Oregon (USA) |
| Oreophoetes peruana (Peruvian Fern Stick Insect) |
| Oriental Brown-sided Skink (Eutropis multifasciatus) |
| Oriental Darter (Anhinga melanogaster melanogaster) |
| Oriental Fire-bellied Toad (Bombina orientalis) |
| Oriental Garden Lizard (Calotes versicolor) |
| Oriental Rat Snake, Common (Ptyas mucosa) |
| orientalis, Antaresia stimsoni (Eastern Blotched Python) |
| orientalis, Bombina (Oriental Fire-bellied Toad) |
| orlovi, Rhacophorus (Orlov's Tree Frog) |
| Orlov's Tree Frog (Rhacophorus orlovi) |
| ornata, Aphaniotis (Malayan Blue-mouthed Lizard) |
| ornata, Ceratophrys (Ornate Horned Frog) |
| ornata, Chrysopelea (Golden Tree Snake) |
| ornata, Musa (Bronze Banana) |
| ornata, Phelsuma (Vinson's Gecko) |
| ornata, Uromastyx (Ornate Spiny-tailed Agama) |
| Ornate Crevice Dragon (Ctenophorus ornatus) |
| Ornate Horned Frog (Ceratophrys ornata) |
| Ornate Nursery Frog (Cophixalus ornatus) |
| Ornate Plated Lizard (Zonosaurus ornatus) |
| Ornate Rainforest Frog (Cophixalus ornatus) |
| Ornate Rock Lobster (Panulirus ornatus) |
| Ornate Soil-crevice Skink (Notoscincus ornatus) |
| Ornate Spiny-tailed Agama (Uromastyx ornata) |
| ornatissima, Chrysopelea ornata (Golden Tree Snake) |
| ornatum, Astrophytum (Monk's Hood Cactus) |
| ornatus, Cophixalus (Ornate Nursery Frog) |
| ornatus, Ctenophorus (Ornate Crevice Dragon) |
| ornatus, Notoscincus (Ornate Soil-crevice Skink) |
| ornatus, Panulirus (Ornate Rock Lobster) |
| ornatus, Varanus (Forest Nile Monitor) |
| ornatus, Zonosaurus (Ornate Plated Lizard) |
| Ornithoptera priamus euphorion (Cairns Birdwing) |
| Ornithorhynchus anatinus (Platypus) |
| Orobanchaceae (Broomrape family) |
| Orobanche sp. (Broomrape) |
| Orpine family (Crassulaceae) |
| Orthoptera (Grasshoppers, Crickets, and Katydids) |
| Orthotomus ruficeps (Ashy Tailorbird) |
| Orthriophis taeniurus (Striped-tailed Rat Snake) |
| Ortolan Bunting (Emberiza hortulana) |
| Oryctolagus cuniculus (European Rabbit) |
| Oryx gazella (Gemsbok) |
| Osteichthyes (see Fishes (Pisces)) |
| Osteoglossiformes (Bonytongues) |
| Osteolaemus tetraspis (Dwarf Crocodile) |
| Osteopilus septentrionalis (Cuban Tree Frog) |
| Ostriches, Emus, etc. (Ratites) (Struthioniformes) |
| Otis tarda (Great Bustard) |
| oustaleti, Furcifer (Madagascar Giant Chameleon) |
| ovalifolia, Verticordia (Featherflower) |
| ovata, Neottia (Common Twayblade) |
| Ovophis okinavensis (Okinawan Habu) |
| Owl, Barred (Strix varia) |
| Owl, Little (Athene noctua) |
| Owl, Spectacled (Pulsatrix perspicillata) |
| Owls (Strigiformes) |
| Oxybelis aeneus (Brown Vine Snake) |
| Oxybelis fulgidus (Green Vine Snake) |
| Oxyuranus microlepidotus (Western Taipan) |
| Oxyuranus scutellatus (Eastern Taipan) |
| Oystercatcher, Pied (Haematopus longirostris) |
| Oysters, Clams, etc. (Bivalvia) |
Sea shell, Perhentian Islands, West Malaysia