In small bowl, combine sour cream, onions, oregano and salt; mix well. Refrigerate until serving time.
In medium bowl with fork or in food processor bowl with metal blade, mash beans well. Add bread crumbs, garlic powder, ground red pepper, pepper and egg; mix well. Mixture will be soft. With wet hands, shape mixture into 4 patties, 1/2 inch thick.
Melt margarine in medium nonstick skillet over medium heat. Cook bean patties 5 minutes. Turn and flatten slightly; cook an additional 5 minutes or until thoroughly heated and set. Serve in buns with sour cream sauce.