Dissolve yeast in 1/2 cup warm water. Place flour and salt in a large bowl and cut in the shortening. Make a well in the center and pour in the yeast. Mix well, adding enough additional water to make a soft dough. Knead briefly on a floured board. The dough should be soft but smooth and elastic. Form into a ball and place in a large oiled bowl. Turn once to grease both sides. Let rise in a warm place until doubled in bulk, about 2 hours. Punch down and pound lightly to deflate dough. Oil an 14" round pizza pan and pat and stretch the dough to fit the pan, leaving a 1/2" rim to hold the filling. Spread the pizza with the tomatoes and cover with slizes of mozarella. Sprinkle with salt and pepper to taste, oregano and olive oil. Bake in a thoroughly preheated 400F oven for about 30 minutes, or until the crust is golden brown and cheese is bubbly. Serves 8. Per serving: 540 Calories (kcal); 25g Total Fat; (41% calories from fat); 20g Protein; 58g Carbohydrate; 51mg Cholesterol; 510mg Sodium Food Exchanges: 3 1/2 Grain(Starch); 2 Lean Meat; 1/2 Vegetable; 0 Fruit; 4 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.