In a small saucepan, gently melt the butter. Mix the melted butter with the garlic, pepper, and oregano. Reserve in a small bowl.
Place the oysters on the half shell right over the hottest part of the grill. The oyster shouldn't be cut loose from the shell as they can slip and fall right through the grill.
Spoon enough of the seasoned butter over the oysters so that some of it will overflow into the fire and flame up a bit.
The oysters are ready when they puff up and get curly on the sides, about 5 minutes.
In a small bowl, mix together the grated Parmesan and Romano cheeses. Top the oysters with the cheese mix and parsley. Serve on the shells immediately.