Preheat oven to 300 degrees In a medium mixing bowl, combine tuna, mayonnaise, garlic powder, onion powder, salt, and pepper. Mix well.
Slice French loaf horizontally through middle and once in half vertically. Place each half, opened up, on a baking sheet. Scoop and smooth tuna mixture evenly on the bottom half of the loaf and place cheddar cheese on the top open half. Sprinkle both open sides with oregano.
Bake subs for about 10 minutes, until cheese is melted, and the bread has reached the desired hardness.
Remove from oven and add pickles to the bottom half on top of the tuna and the tomatoes to the top half with cheese. Close the sub and cut into desired smaller size subs to serve.