Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions until al dente. Drain and set aside, reserving 1/2 cup of the pasta water.
In a large skillet over medium heat, add the olive oil. Remove the casing from the Italian sausage and crumble it into the skillet. Cook until browned and cooked through, about 5-7 minutes.
To the skillet, add the diced onion, garlic, and red bell pepper. Sauté for 3-4 minutes until the vegetables are softened. Add the zucchini and cook for an additional 2-3 minutes.
Pour in the crushed tomatoes and add oregano, basil, red pepper flakes (if using), salt, and pepper. Stir to combine and let it simmer for about 5-10 minutes until heated through.
Add the cooked spaghetti to the skillet, tossing it gently in the sauce. If the pasta seems dry, add reserved pasta water a little at a time until desired consistency is reached.
Plate individual servings and sprinkle with grated Parmesan cheese and fresh basil leaves.
Notes
Feel free to adjust the spice level by choosing mild or spicy sausage.