Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Mix flour, baking soda, and salt together in a large bowl; add oats and sugar. Stir in margarine until well incorporated. Press half the oat mixture into the prepared baking pan. Spread apple butter over oat mixture layer. Sprinkle remaining oat mixture over apple butter layer. Press top layer gently with a spoon.
Bake in the preheated oven until oat mixture is crisp and lightly browned, about 55 minutes.