Heat the oil in a large heavy skillet over medium heat.
Add the celery and saute for about 2 minutes. Add the eggplant and saute for 2 minutes. Add the onions and saute for about 3 minutes. Add the red pepper and saute for about 5 minutes.
Add the diced tomatoes and their juice, raisins and oregano. Simmer over medium-low heat for about 20 minutes. The mixture should thicken.
Stir in the vinegar, sugar, capers, and the salt and pepper. Season with more salt and pepper to taste.
Serve at room temperature, garnished with fresh basil leaves.