Preheat oven to 425 degrees F. Scrub russet potatoes thoroughly with a brush; pat dry. Prick potatoes with a fork and rub with olive oil. Bake for 40 to 60 minutes or until tender. Split in half lengthwise. Carefully scoop out the flesh of each potato, leaving a 1/4-inch-thick shell.
In a bowl mash scooped-out potato flesh; 1 cup shredded mozzarella cheese ; desired fillings and 2 teaspoon dried oregano or Italian seasoning.
Spoon 1/4 cup marinara sauce into the bottom of each shell; top with the potato mixture. Sprinkle each potato with 1 tablespoon shredded mozzarella cheese and 1 teaspoon grated Parmesan cheese. Place on a baking sheet. Bake about 15 minutes or until heated through.