1. To make sauce, combine yogurt, lemon juice, garlic, and salt in a cup. Set aside.
2. To make meatballs, combine ground beef, fresh oregano, salt, garlic, and ground red pepper in a medium bowl just until combined. Form 1-1/2-inch meatballs (about 16) and place on sheet of wax paper.
3. Heat the oil in a large nonstick skillet over medium-high heat. Add the meatballs and cook 4 to 6 minutes, turning until browned on all sides. Fold a paper towel into quarters and press onto bottom of pan with wooden spoon to soak up all fat; discard. Reduce heat to medium. Add 2 tablespoons water, cover and cook 2 to 3 minutes more just until meatballs are cooked through.
4. Fill each pita with 2 meatballs, 1 tablespoon sauce, and lettuce. Makes 4 servings.