Preheat oven to 350 degrees F. Heat oil on med-high heat and add onion, pepper and garlic. Sautee for 2 to 3 minutes, then add hamburger, oregano and Italian seasoning. Salt and pepper to taste. Cook until hamburger is no longer pink. Drain, leaving maybe 2 Tbsp. of fat for flavor. Remove from heat and cool.
Prepare shells as directed on the box. Be sure the shells are cooked until tender. Drain and cool in a soak of cold water (maybe 2 minutes).
Transfer meat to a mixing bowl and add egg, bread crumbs, and 1/2 cup of mozzarella cheese. Mix vigorously until a soft meaty stuffing is formed. Form heaping spoonfuls of stuffing and gently place them inside cooked pasta shells.
Empty jar of spaghetti sauce over the entire pan of stuffed shells, dress with mozzarella cheese, cover, and bake for 40 minutes.