| Name | Attribute | Level | Type |
|---|---|---|---|
| Ocular Triplet Pupile | 60 | Boss Monster | |
| Ocular Triplet Pupile | 100 | Boss Monster | |
| Ocular Triplet Pupile | 80 | Boss Monster | |
| Ocular Triplet Pupile | 80 | Boss Monster | |
| Ocular Triplet Pupile | 40 | Boss Monster | |
| Oculus | - | 1 | General |
| Odorologist Grenouille | - | 1 | General |
| Old Elven Gyuden | - | 1 | General |
| Old Fence | - | 1 | Interactive |
| Old Man | - | 1 | General |
| Old Man | - | 1 | General |
| Old Man | - | 1 | General |
| Old Man Barley | - | 1 | General |
| Omu | - | 1 | General |
| On/Off | - | 1 | Interactive |
| Onami | - | 1 | General |
| Onami | - | 1 | General |
| One-Horned Giant | - | 1 | General |
| One-Horned Giant | - | 1 | General |
| One-Horned Giant | 81 | General Monster | |
| One-Horned Giant | 81 | General Monster | |
| One-Man Army Dominius | 31 | Boss Monster | |
| Onyx Axe Bazoros | 80 | World Boss | |
| Onyx Axe Bazoros | 80 | World Boss | |
| Opaltoad | 49 | Elite Monster | |
| Opaltoad | 49 | Elite Monster | |
| Opaltoad | - | 19 | Elite Monster |
| Opaltoad | 19 | Elite Monster | |
| Oran Grass | - | 1 | Interactive |
| Orc Rogue | 3 | General Monster | |
| Orcan Champion Skarr | 57 | Boss Monster | |
| Orcan Champion Skarr | 57 | General | |
| Orcan Chieftain Shazu | 55 | Boss Monster | |
| Orcan Guard | 53 | Elite Monster | |
| Orcan Guard Captain | 52 | Elite Monster | |
| Orchid | - | 1 | General |
| Orcish Sprite | 55 | General | |
| Orcish Sprite | 55 | General Monster | |
| Ordinary Stone Statue | - | 1 | Interactive |
| Ore Mud Monster | 85 | General Monster | |
| Ore Pile | - | 90 | Interactive |
| Orid's Caravan Meyer | - | 1 | General |
| Osprae | - | 124 | Elite Monster |
| Osprae | - | 150 | Elite Monster |
| Osprae | - | 149 | Elite Monster |
| Osprae | - | 148 | Elite Monster |
| Osprae | - | 147 | Elite Monster |
| Osprae | - | 146 | Elite Monster |
| Osprae | - | 145 | Elite Monster |
| Osprae | - | 144 | Elite Monster |