Set a large pot of salted water to boil. Remove sausage meat from casings and roll into 24 balls. In a large saucepan or Dutch oven, heat oil over medium-high. Add sausage and cook until browned all over, about 5 minutes. Using a slotted spoon, transfer meatballs to a plate. Add onion to pan and cook, stirring frequently, until softened, about 5 minutes. Add tomatoes, breaking them up as you go, and season with salt and pepper; bring sauce to a boil and cook 5 minutes.
Add pasta to water and cook according to package instructions. Meanwhile, add meatballs to sauce and rapidly simmer until sauce is slightly thickened, about 7 minutes. Reserve 8 meatballs and 1 1/2 cups sauce to make Spinach and Meatball Calzones at another time. Drain pasta, then toss with remaining sauce and meatballs.