- Number 96: Dark Mist
- Number 9: Dyson Sphere
- Number C32: Shark Drake Veiss
- Number C39: Utopia Ray
- Number C39: Utopia Ray Victory
- Obelisk the Tormentor
- Obnoxious Celtic Guardian
- Ocubeam
- Odd-Eyes Dragon
- Odin, Father of the Aesir
- Ojama Black
- Ojama Green
- Ojama King
- Ojama Yellow
- Old Vindictive Magician
- One Hundred-Eyed Dragon
- One-Eyed Shield Dragon
- Oni Tank T-34
- Onionman
- Orange Shaman
- Orca Mega-Fortress of Darkness
- Orgoth the Relentless
- Orichalcos Aristeros
- Orichalcos Dexia