Ook-Ook (ウキハムくん) is a hamster who appears in Hamtaro: Rainbow Rescue and in some of the Japanese episodes. He wears a monkey suit and only speaks by going "Ukii! Ukii!". Buster is always chasing after him, to bring him to justice.

At the end of Cheerful Ook-Ook!, Buster says that his desire to bring Ook-Ook to justice was just part of a game they were playing and that they are really good friends.


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