| Name | Attribute | Level | Type |
|---|---|---|---|
| Ovine Overlord Otis | - | 40 | Elite Monster |
| Overloading Electrosphere | - | 61 | Elite Monster |
| Overloading Electrosphere | - | 61 | Elite Monster |
| Outpost Oak's Elven Soldier | - | 1 | General |
| Outpost Human Soldier | - | 1 | General |
| Outer Box 9 | - | 1 | Interactive |
| Outer Box 8 | - | 1 | Interactive |
| Outer Box 7 | - | 1 | Interactive |
| Outer Box 6 | - | 1 | Interactive |
| Outer Box 5 | - | 1 | Interactive |
| Outer Box 4 | - | 1 | Interactive |
| Outer Box 3 | - | 1 | Interactive |
| Outer Box 2 | - | 1 | Interactive |
| Outer Box 12 | - | 1 | Interactive |
| Outer Box 11 | - | 1 | Interactive |
| Outer Box 10 | - | 1 | Interactive |
| Outer Box 1 | - | 1 | Interactive |
| Out of control mercenary | 58 | General Monster | |
| Out of control mercenary | 58 | General Monster | |
| Out of control fighter | 58 | General Monster | |
| Out of control fighter | 58 | General Monster | |
| Out of control cavalry | 58 | General Monster | |
| Out of control cavalry | 58 | General Monster | |
| Otherworldly Goblin Grandmaster | 80 | General Monster | |
| Otherworldly Goblin | 83 | General Monster | |
| Otherworldly Goblin | - | 1 | General |
| Otherworldly Eye Demon | - | 1 | General Monster |
| Otherworld Walker Punishment | 65 | World Boss | |
| Otherworld Walker Crime | 65 | World Boss | |
| Otherworld Judge Frasho | 72 | Boss Monster | |
| Ossifighter Boney Prince Charlie | 56 | Elite Monster | |
| Osprae | - | 55 | Elite Monster |
| Osprae | - | 43 | Elite Monster |
| Osprae | - | 81 | Elite Monster |
| Osprae | - | 24 | Elite Monster |
| Osprae | - | 34 | Elite Monster |
| Osprae | - | 39 | Elite Monster |
| Osprae | - | 42 | Elite Monster |
| Osprae | - | 85 | Elite Monster |
| Osprae | - | 2 | Elite Monster |
| Osprae | - | 13 | Elite Monster |
| Osprae | - | 32 | Elite Monster |
| Osprae | - | 26 | Elite Monster |
| Osprae | - | 58 | Elite Monster |
| Osprae | - | 44 | Elite Monster |
| Osprae | - | 93 | Elite Monster |
| Osprae | - | 101 | Elite Monster |
| Osprae | - | 28 | Elite Monster |
| Osprae | - | 22 | Elite Monster |
| Osprae | - | 25 | Elite Monster |