- Ojama Trio
- Ojama Yellow
- Ojamagic
- Ojamandala
- Ojamuscle
- Old Vindictive Magician
- One for One
- One Hundred Thousand Gauss
- One Hundred-Eyed Dragon
- One-Eyed Shield Dragon
- One-on-One Fight
- One-Shot Wand
- Oni Tank T-34
- Onionman
- Orange Shaman
- Orca Mega-Fortress of Darkness
- Ordeal of a Traveler
- Ordeal of the Hero
- Orgoth the Relentless
- Orichalcos Aristeros
- Orichalcos Deuteros
- Orichalcos Dexia
- Orichalcos Gigas
- Orichalcos Kyutora