Heat olive oil in a large skillet over medium-high heat. Add the Italian sausage and cook for 3 to 4 minutes, breaking it apart as it browns. Add the chopped onion and continue cooking until the sausage is fully cooked through, about 5 to 6 more minutes. Stir in the minced garlic and cook for 1 more minute. Drain off any excess grease.

Transfer the cooked meat mixture to the slow cooker. Add the crushed tomatoes, diced tomatoes, Italian seasoning, oregano, basil, salt, pepper, and water. Stir to combine. Do not add the pasta yet. The sauce will look watery and that is normal. The extra liquid will be absorbed by the pasta later.

Cover and cook on HIGH for 3 hours or LOW for 6 hours. Do not lift the lid during this time.

Stir in the uncooked ziti, pressing all the pasta down into the sauce so it is fully submerged. Cover and continue cooking on HIGH for 15 to 25 minutes. Stir once after the first 15 minutes to help the pasta cook evenly. If your slow cooker was on LOW, expect closer to 25 minutes since the cooker needs time to reheat after adding the cold pasta.

Once the pasta is tender, turn off the slow cooker. Scatter the shredded mozzarella evenly over the top. Cover and let rest for 5 minutes until the cheese melts. Serve hot straight from the crockpot.