1. Obli-Gator
  2. Obsolete Techie
  3. Obtuse Angler
  4. OctoBear
  5. Octopuss in Boots
  6. Offense Attorney
  7. Ogre Achiever
  8. Omnomnomnivore
  9. On-and-off Guard
  10. One-And-A-Halfling
  11. One-armed Archer
  12. One-armed Bandit
  13. One-eyed Beholder
  14. One-hit Wanderer
  15. One-Hit Wonder
  16. One-legged Centipede
  17. Onion Ringleader
  18. Onomatopoet
  19. Open Sorceror
  20. Operetta Phantom
  21. Optical Illusionist
  22. Optimus Slime
  23. Orc Angel
  24. Orc de Triomphe
  25. Orc of the Covenant
  26. Organ Grinder
  27. Origami Dragon
  28. Original Impostor
  29. ORLY Owl
  30. Orthodontisaur
  31. Oscillating Fanboy
  32. Ottoman Umpire
  33. Out-of-Frame Monster
  34. Out-of-service Dog
  35. Out-of-shapeshifter
  36. Out-of-Tuna
  37. Out-Sorcerer
  38. Outsourced Monster
  39. Oval Lord
  40. Over-confident Noob
  41. Overbearing Waiter
  42. Oxy-Moron
  43. Ozone Slayer