Lightly coat the inside of your slow cooker with olive oil.
In a heavy-bottom saucepan over medium-high heat, warm the olive oil. Cook the sausage, breaking into pieces, until well browned (~ 6-8 minutes).
Add the onion, a strong pinch of salt, and a couple of grinds of black pepper. Cook until the onion has softened (~ 5 to 7 minutes). Add the garlic and oregano; cook until fragrant (~ 1 minute).
Reduce heat to medium-low. Add ziti and cook, stirring constantly, until the edges of the pasta become translucent (~ 4-5 minutes)
Off heat, stir in the tomatoes and tomato sauce, scraping up any browned bits. Transfer to the slow cooker. Cover and cook until pasta is tender (~ 2 to 3 hours on high).
Dollop ricotta over ziti and sprinkle with mozzarella. Cover and let sit until cheese is melted (~ 20 minutes). Sprinkle with basil and serve.