Gather the ingredients.
Heat a gas or charcoal grill to high heat. Ideally, the grill should hit 500 F.
In a small saucepan, gently melt 4 ounces (½ cup) softened unsalted butter. Combine the melted butter with 2 tablespoons finely chopped garlic, 1 teaspoon freshly ground black pepper, and 1 pinch dried oregano. Reserve this mixture in a small bowl.
Place 18 large oysters on the half shell directly over the hottest part of the grill. Ensure the oysters are not cut loose from the shell to prevent them from slipping and falling through the grill.
Spoon enough of the seasoned butter over the oysters so that some of it overflows into the fire, causing a small flame. The oysters are ready when they puff up and become curly on the sides, about 5 minutes.
In a small bowl, mix together 1 ounce of grated Parmesan cheese and 1 ounce of grated Romano cheese. Top the oysters with the cheese mix and 2 teaspoons of finely chopped parsley. Serve on the shells immediately.
