Better known as Golden Oozaru form. It is like Oozaru form but it has golden fur and it only appears in the GT series. Golden Oozaru is reached when a SS looks at a full moon (full Earth in Goku's case). From Golden Oozaru they can then turn into SS 4.
The advantage is being super strong (not as strong as SS 4 though). The disadvantages are the same as normal Oozaru which is that if not used to the form then you can't control it and that if you tail is chopped of you can't turn Oozaru again( apart from Goku and others I suppose if they had tried what Goku did).