Preheat the oven to 400˚. Roll out the pizza dough on a floured surface until it’s a very flat, large rectangle, about 11 by 18 inches. Spread with the marinara sauce, leaving a 1-inch border all around, then sprinkle with the parmesan, oregano and salt. Evenly top with the mozzarella slices and half of the pepperoni.
Unwrap the string cheese and arrange it in a line on one long edge of the dough. Starting from that long edge, roll up the dough as tightly as possible so that the string cheese winds up in the middle of the roll. Pinch the seam of the dough to seal, then flip seam-side down.
Slice into 1-inch pieces with a serrated knife and place the rolls cut-side up in a 12-inch round baking dish or cast-iron skillet. Tuck the remaining pepperoni into the folds of the pizza rolls, leaving just a little sticking out. Allow the rolls to rise at room temperature for 20 minutes.
Bake the rolls until the crust is golden and the filling is bubbly, 20 to 25 minutes. Serve with extra marinara sauce for dipping.
