In a Large, heavy skillet, heat the oil over a medium flame. Add the celery and sauté until crisp-tender, about 2 minutes. Add the eggplant and sauté until beginning to soften, about 2 minutes. Add the onion and sauté until translucent, about 3 minutes. Add the red pepper and cook until crisp-tender, about 5 minutes. Add the diced tomatoes with their juices, raisins, and oregano. Simmer over medium-low heat until the flavors blend and the mixture thickens, stirring often, about 20 minutes. Stir in the vinegar, sugar, capers, and ½ teaspoon each of salt and pepper. Season with more salt and pepper to taste. Transfer the caponata to a bowl, garnish with the basil leaves, and serve.