Heat olive oil in a large skillet over medium high heat. Add onion and pepper and saute for about 8 minutes or until tender, stirring occasionally. Next, add the garlic and diced tomatoes and cook for approximately 5 minutes before adding the eggplant, zucchini, and summer squash.
Stir in the salt, pepper and oregano and reduce the heat to low. Cook the stew over low heat for 20 to 30 minutes, or until the vegetables reach your desired tenderness. Add the chopped basil, and more salt and pepper to taste. Enjoy warm or cold!