| Name | Class | XP Level | Timeline | Zone |
| Oakheart | Monk | Pre Searing | Regent Valley, The Northlands | |
| Oakheart | Ranger | Post Searing | Watchtower Coast | |
| Oberans Minion | Warrior | Post Searing | Old Ascalon | |
| Obrhit Barkwood | Warrior | Post Searing | Frozen Forest | |
| Obsidian Behemoth | Ranger | Post Searing | The Underworld | |
| Obsidian Furnace Drake | Elementalist | 24 | Post Searing | Fissure Of Woe |
| Obsidian Guardian | Elementalist | Post Searing | The Underworld | |
| Odelyn The Displeased | Necromancer | Post Searing | Ring Of Fire | |
| Old Red Claw | Elementalist | Post Searing | Snake Dance | |
| Ominous Ooze | Depths Of Tyria | |||
| Onan The Spiritless | Mesmer | Post Searing | Gates Of Kryta | |
| Onaona Kubkub | Warrior | Post Searing | The Great Northern Wall | |
| Oni | 28 | |||
| Oni Keen Oni Talon | 28 | |||
| Oni Of The Deep | 24 | |||
| Optimus Caliph | Elementalist | Post Searing | Abaddons Mouth | |
| Orshad Chieftain | Ranger | Post Searing | The Scar | |
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