Heat the oven to 400°F. Cook penne in water with the tablespoon salt until 2 minutes short of the package time, then drain.
Meanwhile, heat oil in a skillet and cook mushrooms for 7 to 8 minutes until their water evaporates. Transfer to a bowl, melt butter in the skillet and whisk in flour for 1 minute.
Gradually whisk in milk and cream and simmer gently for 3 minutes. Remove from heat and stir in Parmesan, oregano, sauce salt and pepper.
Fold pasta and mushrooms into the sauce and transfer to a deep 9-by-13-inch baking dish. Scatter mozzarella and pepperoni evenly over the top and cover with foil.
Bake for 20 minutes, then uncover for 8 to 10 minutes until hot at 165°F in the center with melted cheese and curled pepperoni edges. Rest for 5 minutes before serving.