Cook the spaghetti until al dente according to package instructions.
In a large skillet, heat olive oil over medium heat.
Add the sliced sausage to the skillet, breaking it up with a spatula.
Once the sausage is browned, add sliced onion and bell peppers to the skillet.
Stir in the minced garlic and cook for another minute.
Pour in the canned diced tomatoes, along with oregano, salt, and pepper.
Simmer the sauce to allow flavors to meld, about 15 minutes.
Combine the cooked spaghetti with the sauce, tossing to coat evenly.
Serve hot, garnished with fresh basil leaves.