Preheat your skillet over medium heat.
Spread olive oil on one side of each slice of bread.
Place one slice of bread, oiled side down, onto the skillet.
Spread pizza sauce on the un-oiled side of the bread in the skillet.
Layer with mozzarella, pepperoni, and Parmesan cheese.
Top with another slice of bread, oiled side up.
Cook until the bottom is golden brown, about 3-4 minutes.
Carefully flip the sandwich and grill the other side.
Remove when both sides are golden brown and the cheese is melted, approximately 3-4 more minutes.
Let the sandwich cool for a minute, then slice and serve.