Heat oil in a large skillet over medium-high heat. Add sausage and cook for 7 to 9 minutes, breaking it into small pieces, until it reaches 160°F; transfer to a plate, leaving about 1 tablespoon fat in the pan.
Add mushrooms and bell pepper and cook for 8 to 10 minutes until softened and the mushroom liquid has evaporated. Stir in garlic for 30 seconds.
Return sausage to the skillet with tomatoes, oregano and pepper. Simmer uncovered for 10 to 12 minutes until thick enough to mound on a spoon.
Heat the broiler. Split the rolls without cutting through the hinge and toast the cut faces for 1 to 2 minutes.
Divide mozzarella over both cut faces of each roll and broil for 1 to 2 minutes until melted. Spoon the hot sausage mixture along the bottoms, keeping loose sauce in the skillet.
Close the subs, rest for 2 minutes and cut across the middle. Serve one sub per person while the cheese still stretches.