Heat olive oil in a large pot over medium heat. Add onion and garlic, sauté until soft and fragrant.
Stir in zucchini and bell pepper, cook for 3-4 minutes.
Add diced tomatoes, vegetable broth, spaghetti, oregano, basil, salt, and pepper. Stir well.
Bring to a boil, then reduce heat and simmer uncovered for 12-15 minutes until pasta is cooked and liquid reduces into a sauce. Stir occasionally.
Serve hot with Parmesan and fresh basil, if using.