Set oven rack 4 inches from broiler and preheat. Line a rimmed baking sheet with foil. Slice bread loaf into thirds, then slice each third in half lengthwise so you have 6 equal pieces.
Flake tuna in a medium bowl. Stir in oil, lemon juice, parsley and olives. Season with salt and pepper.
Spread tuna mixture evenly over each piece of bread and sprinkle with cheese.
Place on baking sheet and broil until cheese is bubbling, 1 to 2 minutes.
