Heat 1/4 cup of the oil in a heavy large skillet over medium heat. Add the celery and saute until crisp-tender, about 2 minutes. Add the eggplant and saute until beginning to soften, about 2 minutes. Season with salt. Add the red pepper and cook until crisp-tender, about 5 minutes. Add the onion and saute until translucent, about 3 minutes. Add the diced tomatoes with their juices, raisins, and oregano. Season with salt and pepper, to taste. Simmer over medium-low heat until the flavors blend and the mixture thickens, stirring often, about 20 minutes. Add the vinegar, sugar, and capers. Season with salt and pepper, to taste.