Beat together butter, sugar, lemon peel, and vanilla until creamy. Add flour and oats; mix well. Cover; chill in refrigerator about 30 minutes. Heat oven to 350°F . Shape dough into 1-inch balls. Place 3 inches apart on an ungreased cookie sheet. Flatten with bottom of glass dipped in sugar. Bake 12 to 15 minutes or until edges are light golden brown. Remove from oven. Cool 1 minutes on cookie sheet; remove to wire rack. Cool completely. If desired, sprinkled with powdered sugar. Store in tightly covered container.