O (often misunderstood as 0 or Zero) is a thief encountered in Castlevania: Harmony of Dissonance. When defeated it will flee by jumping away, always leaving either a random sub-weapon or a "Big Heart". This enemy is the only way to unequip sub-weapons, which can allow Juste to use his healing spell by using the Summoning Tome. In Maxim mode, it will rarely drop anything at all, since sub-weapons are not used in it. Instead, it may drop a single heart.
An alternate version of this enemy is the Mimic (or Manemane), which mimics a player's behavior. It appears in a couple of rooms in the Skeleton Cave (Castle A). There is even a minor puzzle that requires to lead a Mimic to an unreachable switch (both the Mimic and the switch are located in the "closed off" top section of the room), by having it mirror Juste's movements. The Mimic does not steal any items.
|Enemy Data: O|
|Image||Name - Game|
| ''Abil: As its name implies, it mirrors (or Mimics) the players movements.|| Weak: Ice|
| Common Drop: Potion|
Rare Drop: Brigandine
| Castle A: Skeleton Cave|
| ''Abil: Jumps away after it is defeated, dropping a random sub-weapon (or Big Heart) as it flees.|| Level: 19|
| Drop: It will also drop either a random Sub-Weapon or a Big Heart when defeated.|
Common Drop: Drumstick
Rare Drop: Noble Robe
| Castle B: Marble Corridor, Room of Illusion|
- O (Ou) is probably a parody of O-jirō, a character from the Obake no Q-tarō series.
- Mimic's Japanese name Manemane. Japanese 真似 (Mane) - translation Mimicry.