O¶
- O - Oversoul
- Obelisk the Tormentor
- Obnoxious Celtic Guard
- Obsidian Dragon
- Ocean Dragon Lord - Neo-Daedalus
- Ocean’s Keeper
- Octoberser
- Ocubeam
- Offering to the Immortals
- Offering to the Snake Deity
- Offerings to the Doomed
- Ogre of the Black Shadow
- Ogre of the Scarlet Sorrow
- Oilman
- Ojama Black
- Ojama Blue
- Ojama Country
- Ojama Delta Hurricane!!
- Ojama Green
- Ojama King
- Ojama Knight
- Ojama Red
- Ojama Trio
- Ojama Yellow
- Ojamagic
- Ojamuscle
- Old Vindictive Magician
- Omega Goggles
- Ominous Fortunetelling
- One-Eyed Shield Dragon
- One for One
- Oni Tank T-34
- Ooguchi
- Ookazi
- Oppressed People
- Opti-Camouflage Armor
- Opticlops
- Option Hunter
- Oracle of the Sun
- Orb of Yasaka
- Orbital Bombardment
- Orca Mega-Fortress of Darkness
- Ordeal of a Traveler
- Order to Charge
- Order to Smash
- Oscillo Hero
- Oshaleon
- Otherworld - The “A” Zone
- Otohime
- Outstanding Dog Marron
- Over Destiny
- Over Limit
- Overdoom Line
- Overdrive
- Overdrive Teleporter
- Overload Fusion
- Overwhelm
- Overworked
- Owner’s Seal
- Oxygeddon
- Oyster Meister