Clean the mushrooms by removing the stems then thoroughly wiping them clean with a damp cloth or paper towel.
Preheat the oven to 375℉ and spray a baking dish with olive oil or a non-stick cooking spray.
In a large mixing bowl, stir together all the filling ingredients: breadcrumbs, romano cheese, garlic, clams, clam juice, green onion, egg, butter, parsley, and oregano. Mix well to evenly combine all ingredients.
Place the stuffing mixture inside each mushroom cavity and slightly mound it for a generous fill.
Arrange the stuffed mushrooms in the lightly oiled baking dish, ensuring they are not touching.
Cover the baking dish with foil and bake for 20-25 minutes, until the mushrooms are tender.
Remove the foil and sprinkle each mushroom with the grated mozzarella cheese, then bake for an additional few minutes until the cheese is golden and bubbly. Serve warm.