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, in attempt 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.