Spend 10 minutes prepping, 30 minutes cooking, and you've got a delicious weeknight dinner.
Preheat the oven to 400°. Bring a large pot of water to a boil. In a large skillet, heat the oil over medium-high. Add the mushrooms, onion, and garlic and cook, stirring often, until softened, about 5 minutes.
Add the beef and cook, stirring and breaking up the meat with a spoon, until browned, about 5 minutes. Add the marinara. Simmer for 5 minutes more; season with salt and pepper.
Salt the boiling water and add the pasta. Cook until al dente. Reserve 1/2 cup of the cooking water and drain the pasta. Return the pasta and cooking water to the pot. Stir in the sauce and chopped oregano, then 3 cups mozzarella. Transfer to a greased 9-by-13-inch baking dish.
Bake the ziti for 10 minutes. Top with 1 cup Parmesan and the remaining 1 cup mozzarella. Bake until bubbling, 15 to 20 minutes more. Top the ziti with oregano leaves and more Parmesan.