Preheat the oven to 375 degrees F and lightly grease a 9x13 baking dish.
Bring a large pot of salted water to a boil and cook the ziti according to package directions until just al dente. Drain and set aside.
Heat the olive oil in a large skillet over medium heat. Add the sausage and cook, breaking it up with a spoon, until browned.
Add the diced onion, sliced peppers, and minced garlic to the skillet. Cook for 5 minutes until the vegetables soften.
Stir in the marinara sauce, Italian seasoning, salt, and black pepper. Simmer for 3 minutes.
Combine the drained ziti with the sausage mixture in the skillet or a large bowl.
Spread half the pasta mixture into the prepared baking dish. Sprinkle with half the mozzarella and half the Parmesan.
Add the remaining pasta mixture on top, then finish with the rest of the cheeses.
Bake for 20 to 25 minutes until the cheese is bubbly and lightly browned on top.