In a shallow bowl whisk together the olive oil and vinegar. Marinate the tomatoes in the mixture, stirring occasionally.
Meanwhile in a small bowl combine mayonnaise, parsley, oregano, black pepper and 4 teaspoons Parmesan cheese. Spread mixture on each slice of toasted bread. Place marinated tomatoes on 2 slices and sprinkle with remaining Parmesan cheese.
Place on a baking tray and grill for 5 minutes or until cheese turns golden brown. Serve immediately, open faced or closed.