Boil the ziti in salted water until al dente, approximately 8 minutes.
Drain and set aside.
Heat olive oil in a large skillet over medium heat.
Cook the sausage with the onion until browned.
Add garlic and stir for a minute more.
Pour in the crushed tomatoes, seasoning with salt, pepper, and oregano.
Let simmer for 15 minutes, occasionally stirring.
Preheat your oven to 375°F (190°C).
Mix the cooked ziti, sausage sauce, and mozzarella together in a large bowl.
Transfer the mixture to a baking dish.
Sprinkle with Parmesan and bake for 25 minutes or until bubbly and golden.