| Name | CR | Type | Environment |
| Obambo | 13 | undead (incorporeal) | warm forests |
| Oblivion | 20 | ooze | any (Negative Energy Plane) |
| Occulari | 7 | aberration (swarm) | any underground |
| Oceanid | 7 | fey (water) | any oceans |
| Ochre Jelly | 5 | ooze | temperate underground or marshes |
| Octopus | 1 | animal (aquatic) | temperate or cold ocean |
| Octopus, Giant Octopus | 8 | animal (aquatic) | any ocean |
| Ogre | 3 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Boss | 7 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Brute | 6 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Destroyer | 10 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Glutton | 11 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Hunter | 9 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre King | 13 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Mancatcher | 8 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Priest | 4 | humanoid (giant) | temperate or cold hills |
| Ogre, Ogre Stalker | 4 | humanoid (giant) | temperate or cold hills |
| Ogrekin | 2 | humanoid (giant) | any |
| Ogrekin, Kin Seeker | 6 | humanoid (giant, orc) | any |
| Ogrekin, Kreegwood Stalker | 3 | humanoid (giant) | any |
| Ogrekin, Stroud Apprentice | 10 | humanoid (giant) | any |
| Ogrekin, Thanegraves Looter | 4 | humanoid (cold, giant) | any |
| Ohancanu | 5 | fey | temperate forests |
| Oma | 16 | magical beast | gas giants or vacuum |
| Oneirogen | 2 | outsider (native) | any land |
| Oni, Atamahuta | 11 | outsider (oni, giant, native, shapechanger) | temperate mountains |
| Oni, Earth Yai | 13 | outsider (giant, native, oni, shapechanger) | temperate mountains |
| Oni, Fire Yai | 15 | outsider (fire, giant, native, oni, shapechanger) | temperate or warm hills or mountains |
| Oni, Ice Yai | 14 | outsider (cold, oni, giant, native, shapechanger) | cold mountains |
| Oni, Ja Noi | 5 | outsider (goblinoid, native, oni, shapechanger) | temperate or warm hills |
| Oni, Kigyo | 6 | outsider (aquatic, native, oni, shapechanger) | any water |
| Oni, Kuwa | 4 | outsider (human, native, oni, shapechanger) | any |
| Oni, Najikai | 8 | outsider (native, oni, reptilian, shapechanger) | warm forests or mountains |
| Oni, Nogitsune | 7 | outsider (kitsune, native, oni, shapechanger) | any urban |
| Oni, Ogre Mage | 8 | outsider (giant, native, oni, shapechanger) | cold hills |
| Oni, Spirit Oni | 2 | outsider (native, oni) | any |
| Oni, Void Yai | 20 | outsider (giant, native, oni, shapechanger) | cold or temperate mountains |
| Oni, Water Yai | 18 | outsider (aquatic, giant, native, oni, shapechanger, water) | any water |
| Oni, Wind Yai | 16 | outsider (air, giant, native, oni, shapechanger) | temperate mountains |
| Oni, Yamabushi Tengu | 5 | outsider (native, oni, shapechanger, tengu) | temperate mountains |
| Onkushu | 13 | outsider (extraplanar, fire, good) | any (Plane of Fire) |
| Onwu Azu | 1 | animal (aquatic) | warm oceans or rivers |
| Onyvolan | 3 | monstrous humanoid | any urban |
| Onyx Scourge | 16 | outsider (earth, elemental, extraplanar) | any underground (Plane of Earth) |
| Orang-pendak | 1/2 | humanoid (sasquatch) | warm forests |
| Orc | 1/3 | humanoid | temperate hills, mountains, or underground |
| Orc, Gorthek | 7 | animal | temperate hills and plains |
| Orc, Gorthek Rider | 6 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Chieftain | 9 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Lieutenant | 4 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Mystic | 2 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Scout | 5 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Sergeant | 2 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Thug | 3 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc War Drummer | 3 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Warlord | 6 | humanoid (orc) | temperate hills, mountains, or underground |
| Orc, Orc Witch Doctor | 8 | humanoid (orc) | temperate hills, mountains, or underground |
| Oread | 1/2 | outsider (native) | any land |
| Orphne | 4 | fey | Any underground or urban |
| Orsheval | 4 | outsider (extraplanar, lawful) | any (Axis) |
| Osirion Mummy | 6 | undead (augmented humanoid) | — |
| Ossumental, Ossumental (Air) | 12 | undead (air) | warm deserts (Osirion) |
| Ossumental, Ossumental (Earth) | 12 | undead (earth) | warm deserts (Osirion) |
| Ossumental, Ossumental (Fire) | 12 | undead (fire) | warm deserts (Osirion) |
| Ossumental, Ossumental (Water) | 12 | undead (water) | warm deserts (Osirion) |
| Ostovite | 1 | magical beast (extraplanar) | any (Abyss) |
| Ostrich | 1 | animal | warm deserts or plains |
| Otyugh | 4 | aberration | any underground |
| Ouroboros | 21 | magical beast (extraplanar) | any (Astral Plane) |
| Owb | 6 | outsider (extraplanar) | any land or underground (Plane of Shadow) |
| Owb, Owb Prophet | 13 | outsider (extraplanar) | any land or underground (Shadow Plane) |
| Owl, Giant Owl | 5 | magical beast | temperate forests |
| Owl, Great Horned Owl | 1/2 | animal | temperate forests |
| Owlbear | 4 | magical beast | temperate forests |
| Ozimat | 11 | outsider (earth, elemental, extraplanar, incorporeal) | any (Plane of Earth) |