Heat a grill to medium heat. Crumble meat into a large bowl. Add oregano, chili powder, onion, cumin, salt, pepper, and garlic powder. Mix well, and form into patties, each about 3/4 inch thick. Place burgers on grill. Grill, covered for 5 to 7 minutes per side or until done. Place a piece of Monterey Jack cheese on each burger during the last 1 or 2 minutes of grilling, if desired. Serve burgers in buns.