An Oozaru is a monstrous, gigantic ape-creature. Saiyans who retain their tails transform into Oozaru when exposed to Brute Waves measured at 1700 Xeno or more; the full moon is the most typical source of this energy. The Oozaru form has ten times more power than they do as a normal Saiyan, and they can fire blasts of energy from their mouth.
Most Saiyans become mindless engines of destruction in this form. For this very reason, the Saiyans would often wait until a full moon to invade a planet. Saiyans can learn to control their actions in the Oozaru form with practice; Vegeta, for example, was capable of this.