Brush one side of each bread slice with olive oil. Place 3 slices, oil side down, on a 10- by 15-inch baking sheet and layer evenly with basil leaves, tomato slices, and mozzarella slices. Sprinkle lightly with salt and pepper and top with remaining bread slices, oil side up.
Broil sandwiches 6 inches from heat, turning once, until bread is golden brown and cheese is melted, 2 to 3 minutes total. Serve immediately.
Nutritional analysis per sandwich.