Combine uncooked meat, beans, chili powder, and oregano. Spoon into shells; place in a greased 9-inch by 13-inch baking dish. Combine water and picante and pour over shells. Cover; refrigerate overnight.
Remove dish from the refrigerator 30 minutes before baking. Cover and bake at 350 degree F for 1 hour. Uncover; top shells with sour cream, cheese, olives, and onions. Bake 5 to 10 minutes longer.