Oozaru is the giant monkey level that all Saiyan's (with their tails not cut off) can achieve from birth. Saiyan's reach this level by looking at a full moon. This level increases their power by times 10. When a Saiyan's tail is chopped off they can never turn into Oozaru again (Goku kind off is an exception which you will find out if you watch GT episodes).
Advantages are obviously having 10 times your normal power. The disadvantages are if your are not used to the level you can't control yourself and that if your tail is chopped of then you can't turn Oozaru again (see above). If your tail is only pulled out though it will grow back again in about a month.