Heat olive oil in a heavy bottomed skillet, preferably cast iron, over medium high heat. Add onion and cook until translucent, about 3-5 minutes. Add eggplant and salt and cook until it starts to soften. Cook for about 10-12 minutes, stirring frequently, until eggplant is softened. If eggplant is too dry, add additional olive oil.
Stir in tomatoes, capers, green olives and raisins and cook for an additional 3-4 minutes, allowing all the flavors to meld.
Remove from heat, let cool slightly, adjust salt to taste, and garnish with oregano. Serve warm as side dish or let cool completely and serve cold as antipasti.