"Oozaru" is the term used to describe the giant ape-like creature that Saiyans transform into during a full moon, provided they still have a tail. An Oozaru will always portray evil and destructive characteristics even in the cases of the good Saiyans and Halfbreed Saiyans.
An alternative and far stronger type of Oozaru, known as a Golden Oozaru, can be seen in the Dragon Ball GT series.