Assemble the sandwiches by arranging some sliced mozzarella and basil, sprinkled with salt and pepper, between 2 slices of bread. Repeat for the remaining 2 slices of bread. Brush both sides of the sandwiches with olive oil. Put on hot grill and cook until the cheese is evenly melted, about 1 to 2 minutes on each side. Transfer to serving plates and serve hot.