Oozaru is a fictional transformation in the world of Dragonball, seen in Dragonball, Dragonball Z, and Dragonball GT. The transformation evolved in role from being a transformation available only to Son Goku in the original Dragonball, to a transformation frequently used by the race of Saiyans in Dragonball Z and GT.
 The transformation
When sunlight is reflected by the moon, it contains green spectrum radiation, as it is called in the manga. During a full moon, this radiation exceeds 17 million zeno units per second, which is required to allow an Oozaru transformation. When a Saiyan absorbs the full amount through their eyes, it triggers a reaction in their tails, allowing their transformation into the Oozaru form. A very select number of Saiyans can also transform without the presence of the moon. These particular Saiyans, can compress a planets' atmosphere into a small power ball, which perfectly resembles a full moon. This technique however, comes at a cost, as it drains a portion of the user's ki. In the manga, Vegeta is the only Saiyan shown capable of performing this technique. In the non canonical movie series, another Saiyan named Turles is introduced, who is also seen utilizing the artificial moon. An Oozaru transformation can be canceled in several ways, for example: destroying the (articial) moon, or amputating their tail, which is required to maintain the transformation.
When a Saiyan transforms into this stage, their power increases exponentially,about tenfold according to Vegeta. The increased power level means that a Saiyan can now perform attacks with drastically more force behind them. They can also perform several ki techniques, such as blasting ki from the mouth. Saiyans occasionally pass out after reverting to their normal form, as the transformation can be very exhausting. These effects can potentially be removed though, as the Saiyan spends more time in his transformed state.
An Oozaru bears great resemblance to a giant monkey. Its characteristic features are the present of brown fur on its body and red eyes. Untrained, lower-class Saiyans, such as Goku and Gohan completely lose control while in this form, and go berserk, wrecking destruction upon their surroundings. However, as seen when Vegeta transforms in the Saiyan Saga, trained Saiyans exercise complete control over this form at all times.