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