Preheat oven to 350 degrees. Place washed potatoes on a foil sheet placed directly on the oven rack. Roast potatoes for approximately 1 1/2 hours or until soft and indent when lightly squeezed.
Using a fork, pierce the potatoes lengthwise down the center and horizontally across the top at about 1/2-inch intervals. Squeeze the potato until the filling, "pops" up fluffy.
Spoon about 2 tablespoons of pizza sauce on top each potato. Add mozzarella, tomatoes, and Parmesan. Broil for about 2 minutes to melt cheese. Sprinkle with oregano.