Preheat the oven to 400°F (200°C).
In a 9x13 inch baking dish, combine the sliced bell peppers, onions, and minced garlic.
Drizzle olive oil over the vegetables and season with dried oregano, salt, and black pepper; toss until evenly coated.
Nestle the Italian sausages into the vegetable mixture in a single layer.
Bake for 25 to 30 minutes, rotating the sausages halfway through, until the vegetables are softened and sausages reach an internal temperature of 160°F (71°C).
Remove the dish from the oven and distribute the shredded mozzarella and parmesan cheese evenly over the top.
Return the dish to the oven for 5 minutes, or until the cheese is melted and beginning to brown.
Let rest for 5 minutes before serving.