Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Heat oil in a skillet over medium. Sauté onion until soft, 4–5 minutes. Add garlic and sausage; brown and break up, about 6 minutes. Drain if needed.
Stir marinara and oregano into the sausage. Simmer 3 minutes to meld flavors. Turn off heat.
In a large bowl, mix hashbrowns, eggs, milk, Parmesan, salt, and pepper. Fold in half the mozzarella.
Spread half the hashbrown mix into the dish. Spoon on the sausage-marinara, then top with remaining hashbrowns. Sprinkle remaining mozzarella.
Bake 35–40 minutes until golden and bubbly. Let sit 5 minutes before cutting.