Unexpected pleasures. This holiday season, treat your guests to an untraditional buffet

THIS holiday season, try substituting the unexpected for the expected. Wouldn't it be fun to invite guests to a holiday buffet and let them sample untraditional preparations of foods associated with Thanksgiving and Christmas? Foods like squash, oysters, ham, chicken or turkey, cranberries, and mincemeat.

Casseroles are perfect for buffets. They hold the heat and are convenient for serving. And they're easy on you and your guests. So instead of plain scalloped oysters that have been associated with holiday feasts since colonial times, try an Oysters Rockefeller Casserole, a variation of Oysters Rockefeller. And instead of baked ham or turkey, choose an oven-to-table casserole recipe for one of these favorites.

As accompaniments, Baked Cranberry Sauce and Oscar of the Waldorf Bread add just the right touch. And Chocolate-Mincemeat Cheesecake Bars will surprise and please guests who are expecting the old standbys - pumpkin or mince pie - for dessert. As an alternative or in addition, you may wish to serve something lighter, such as a mousse, whip, or souffl'e.

Much of the shopping for this buffet can be done in advance, and many of the dishes prepared ahead, even to setting and decorating the table.

And to start off this buffet, try a Pumpkin Bisque instead of putting the pumpkin into a pie. Chilled Pumpkin Bisque 2 16-ounce cans pumpkin (not pumpkin pie filling) 2/3 cup sour cream dip with chives 4 cups chicken broth 2 cups light cream or evaporated skim milk Salt to taste Sour cream or yogurt (garnish)

In bowl combine pumpkin, sour cream dip, chicken broth, cream, and salt. Beat smooth with rotary beater. Cover and chill. Serve garnished with dollops of sour cream or yogurt. Serves 10 to 12. Oysters Rockefeller Casserole 1 quart raw oysters 1/4 pound butter or margarine 1 onion, chopped 1 stalk celery, chopped 1/2 cup minced parsley 1 10-ounce package chopped frozen spinach, thawed 1/4 cup Worcestershire sauce 1/2 cup bread crumbs Salt, pepper, and cayenne to taste 1 cup grated Cheddar cheese Bread crumbs

Drain oysters. Saut'e celery and onion in butter for 5 minutes. Add parsley, thawed spinach, Worcestershire, bread crumbs, salt, pepper, cayenne. Grease shallow casserole. Arrange oysters in one layer only. Cover with spinach mixture. Bake at 450 degrees F. for 30 minutes. Remove and sprinkle with cheese and a very thin layer of bread crumbs. Return to oven for 10 minutes. Serves 8. Old South Sweet Potato Casserole 6 yams, cut in 1/2-inch slices 1/2 cup butter or margarine 3 tablespoons cornstarch 3/4 cup sugar 1/2 teaspoon salt 2 cups water 3 tablespoons lemon juice 1 teaspoon grated lemon peel 6 large apples, peeled and sliced 2 cups cubed or sliced cooked ham

Cover yams with water and boil for 20 minutes. Meanwhile, melt butter. Stir in cornstarch, sugar, and salt. Cook together for a minute or so and stir in water, lemon juice, and lemon peel. Cook together until thick and stir in raisins.

Grease 13-by-9-inch glass pan with butter and cover bottom with apple slices. Layer cooked yam slices atop apple slices. Cover yams with the ham. Pour sauce over all and bake at 350 degrees F. for 1 hour. Serves 8. Curried Turkey Pie 1/4 cup light raisins Boiling water 1 1/2 cups herbed stuffing mix 1/4 cup butter or margarine, melted 2 tablespoons water 1/2 cup milk 1 103/4-ounce can condensed cream of celery soup 2 cups cubed cooked turkey or chicken 1/2 cup cooked or canned peas 1 2-ounce can chopped mushrooms, drained 1 tablespoon finely chopped onion 2 teaspoons curry powder (or to taste)

In small bowl, cover raisins with boiling water. Let stand 5 minutes. Drain and set aside. Combine herbed-seasoned stuffing mix, melted butter, and the 2 tablespoons water. Reserve 1/3 cup of mixture, and press remaining into 9-inch pie plate to form a pie shell.

Blend milk into soup. Stir in turkey or chicken, peas, mushrooms, onion, curry powder, and drained raisins. Turn mixture into stuffing-lined pie plate. Sprinkle reserved stuffing mixture over pie. Bake, uncovered, at 375 degrees F. until pie is heated through, 30 to 35 minutes. Serves 6. Oscar of the Waldorf Quick Bread 3 1/2 cups fork-stirred all-purpose flour 3 teaspoons baking powder 1/4 teaspoon salt 3/4 cup sugar 2 large eggs 1/2 cup canned or bottled unsweetened apple juice 1/2 cup real mayonnaise (not imitation) 1 teaspoon vanilla 1 cup unpeeled diced red apple 1/4 cup finely diced celery 3/4 cup coarsely broken walnuts

Grease and flour 2 loaf pans. In large bowl stir together flour, baking powder, salt, and sugar. In small bowl beat eggs slightly. Add apple juice, mayonnaise, and vanilla. Beat until blended. Add to flour mixture with apple, celery, and walnuts. Stir just until flour is moistened. Turn into prepared pans.

Bake in preheated 350-degree F. oven 45 to 50 minutes. Tops will be cracked. Cool in pans on wire rack for 10 minutes. Loosen edges and turn out on rack. Cool completely. Store overnight in tightly closed plastic bags before cutting. Do not try to slice thin. Store any leftover in refrigerator. Baked Cranberry Sauce 1 pound cranberries 1 12-ounce jar (1 cup) orange marmalade 1 cup chopped pecans 1 cup shredded coconut 3/4 cup sugar 1/2 cup water

Combine all ingredients. Spread in 13-by-9-by-2-inch baking dish. Bake, uncovered, in a 350-degree F. oven for 30 minutes. Makes about 4 cups. Chocolate-Mincemeat Cheesecake Bars 1 12-ounce package semisweet chocolate chips 1/2 cup butter or margarine 2 cups graham cracker crumbs 1 cup chopped nuts 1 8-ounce package cream cheese, softened 1 14-ounce can sweetened condensed milk (not evaporated milk) 1 egg 1 teaspoon vanilla 1 9-ounce package condensed mincemeat, crumbled

Preheat oven to 350 degrees F. In large saucepan, over low heat, melt chips and butter. Stir in crumbs and nuts. Press half the mixture firmly onto bottom of greased 15-by-10-inch jelly roll pan (layer will be thin).

In large mixer bowl, beat cheese until fluffy. Beat in condensed milk, egg, and vanilla. Add mincemeat. Mix well. Pour into prepared pan. Top with remaining crumb mixture.

Bake 25 to 30 minutes or until wooden pick inserted in center comes out clean. Cool to room temperature. Chill before cutting into bars. Refrigerate leftovers. Makes 48 bars.