That familiar round carton of oats has probably been in your cupboard for as long as you can remember. But, if you've been using the contents only to make hot cereal and oatmeal cookies, it's time to start tapping the potential of this wholesome food.

Oats are a versatile, economical cooking and baking ingredient. They add a hearty texture and nutty flavor to a variety of baked goods. Toasted oats can be substituted in some recipes for bread crumbs, offering more nutrition; in others, they can replace nuts, providing fewer calories. Oats are also useful as an extender, enabling you to stretch ground beef further in meat loaf and hamburger recipes. Another popular use for oats is in homemade granola, which is far less costly than the store-bought varieties.

Like the wheat berry, the oat is a whole grain comprised of three parts: the germ, the endosperm and the bran. The first two provide protein and vitamins; the bran is a source of natural fiber. In the past few years, there has been renewed interest in fiber as an antidote to over-refined foods, and oats are extremely valuable in restoring fiber to the daily diet.

The oats most commonly found in stores and in recipes are the rolled variety. Since they are unrefined, their nutrients and fiber remain intact. These should not be confused with steel-cut oats, which are more nutritious as a breakfast food, but simply not as practical for use in cooking or baking.

Both "old fashioned" and "quick cooking" oats provide the same nutritional benefits. In most recipes you can use either one. If, however, a recipe does specify old fashioned oats, and you want to substitute the quick cooking variety, use slightly less than the amount called for. Conversely, if you have old fashioned oats on your shelf, and a recipe prescribes the quick cooking kind, you should use about one-quarter cup more than the amount indicated.

If you're ready to expand your horizons beyond hot oatmeal and oatmeal cookies, here are some recipes to get you started.(In each one, you start with uncooked oats.) OATMEAL BREAKFAST SCONES (Makes 8) 1 1/2 cups flour 1 tablespoon baking powder 1 teaspoon cream of tartar 1/2 teaspoon salt 1 1/2 cups old fashioned oats (or 1 1/4 cups quick oats) 1 egg 1/3 cup milk 1/2 cup melted butter 1/4 cup honey 1/2 cup raisins or currents

Sift together the flour, baking powder, cream of tartar and salt into large bowl; add oats. Combine egg, milk, butter and honey, and add to bowl, stirring just until dry ingredients are moistened. Stir in the raisins or currants. Form dough into a ball and turn out onto a lightly floured surface. (Dough will be slightly sticky.) With your hands, form dough into an 8-inch circle. With sharp knife, cut into 8 wedges. Place wedges about 1 inch apart on a greased cookie sheet, and bake in preheated 425-degree oven for 10 to 12 minutes, or until golden-brown. Serve hot with butter and preserves. GARDEN MEAT LOAF (4 servings) 1 pound ground round 2 eggs 1/2 cup shredded or finely chopped celery 1 small onion, finely chopped 1 cup shredded or grated carrots 1/2 cup ketchup 1 1/2 cups old fashioned or quick oats 1/2 teaspoon garlic salt 1/2 teaspoon dry mustard 1 tablespoon worcestershire sauce

Place meat and eggs in large bowl, and work them together with your hands. Add remaining ingredients, mixing thoroughly. Form the mixture into a loaf, and place in a shallow baking pan; or press mixture into an 8 1/2-by-4 1/2 inch loaf pan. Bake in preheated 350-degree oven for 50 minutes. Let the meat loaf cool for 10 minutes before slicing. CRUNCHY TUNA SALAD (4 to 6 servings) 1 cup old fashioned or quick oats 2 7-ounce cans water-pack tuna, well drained 1 salk celery, finely chopped 1 small onion, finely chopped 1 cup mayonnaise 1 to 2 tablespoons lemon juice

Toast the oats by placing them on a cookie sheet; bake in preheated 350 degree oven 15 minutes, or until golden-brown. Let cool thoroughly. In medium bowl combine remaining ingredients, mixing well. Add toasted oats. Correct seasoning, adding more lemon juice if desired. Serve on beds of lettuce, garnished with fresh vegetables, or use as a sandwich spread. OATMEAL JAM BARS (Makes 2 dozen) 1 cup butter or margarine, softened 1 cup light brown sugar 1 3/4 cups flour 1 teaspoon cinnamon 3/4 teaspoon salt 1/2 teaspoon baking soda 2 cups old fashioned or quick oats 1 cup strawberry or raspberry jam

In large bowl, with electric mixer at medium speed, cream butter and sugar until light and fluffy. Sift together flour, cinnamon, salt and baking soda; add to butter mixture, beating at low speed. With wooden spoon, stir in the oats. Reserve 2 cups of the mixture for the topping. Spread the remaining mixture on the bottom of a well greased 9-by-13-by-2-inch baking pan. Spoon jam on top of oatmeal mixture to within 1/2 inch of the outside edges. (Do not try to spread jam, or it will tear the oatmeal base.) Sprinkle reserved 2 cups of mixture over jam. Bake in preheated 400-degree oven 25 to 30 minutes, or until golden-brown. Let pan cool thoroughly on wire rack. With sharp knife cut into bars about 3-inches long by 1 1/2 inches wide.