Preheat broiler on low. On a non-stick cookie sheet, arrange 25 slices of cocktail sourdough bread squares and brush one side lightly with extra virgin olive oil. Toast under broiler for approximately 3 minutes each side or until toasted lightly. Transfer toasts to serving plate.
In a medium bowl, gently toss together the tomatoes, onions, garlic, capers and olives. In a separate bowl whisk together 3 tablespoons of the olive oil, the 3 tablespoons of balsamic vinegar and the oregano.
May refridgerate two mixtures until ready to serve. When ready to serve, pour oil/vinegar mixture over tomato mixture and toss together gently. Season to taste with salt and freshly ground pepper.
Serve cold in a bowl alongside the cocktail toasts and a small bowl of freshly grated parmesan cheese for topping.