Preheat oven to 400°F. Place the whole wheat bread on a rimmed baking sheet. Bake in preheated oven for 15–18 minutes, or until lightly toasted. Remove from the oven and set aside.
Meanwhile, in a small bowl, whisk together the Mazola corn oil and balsamic vinegar.
Place the tuna, red pepper, red onion, parsley, oregano, and salt in a large bowl. Pour in the corn oil and vinegar mixture. Toss gently to combine.
Divide the tuna mixture evenly between the six slices of toasted bread. Top each with a slice of low-fat mozzarella.
Bake for 5–7 minutes, or until the cheese is melted. Serve immediately.