Heat a large skillet over medium heat and add olive oil until shimmering.

Sauté diced onion and minced garlic until onion becomes translucent, about 3 minutes.

Add the ground sausage, breaking it apart with a wooden spoon, and cook until browned and crumbly. Drain excess grease.

Stir in marinara sauce and a splash of water or broth; simmer for 5 minutes to meld flavors.

Boil salted water in a large pot, cook ziti until just shy of al dente (about 7 minutes), then drain.

Combine pasta with sausage‑sauce mixture in a large baking dish; top with mozzarella, Parmesan, oregano, red pepper flakes, salt, and pepper.

Cover with foil and bake at 375°F (190°C) for 20 minutes. Remove foil and bake another 5 minutes until cheese is bubbly and golden.

Let rest for a few minutes before serving.