Fold two aluminum foil strips and line the slow cooker perimeter. Spray with oil.

Heat oil in a Dutch oven over medium-high heat. Brown sausage, about 6-8 mins.

Add onion and cook until lightly browned, about 5 mins.

Stir in garlic, oregano, salt, and pepper; cook until aromatic, about 1 min.

Reduce heat to medium-low. Add ziti and stir until edges are translucent, about 4 mins.

Remove from heat, mix in crushed tomatoes and tomato sauce, scraping browned bits.

Transfer mixture to slow cooker, cover, and cook on LOW for 3 hrs.

Remove foil, gently stir pasta, dollop ricotta, and sprinkle mozzarella.

Cover and let sit for 20 mins to melt cheese. Garnish with basil before serving.