These bars make a delicious afternoon snack during holiday time.
Heat oven to 375 degrees. In a large bowl, whisk together flour, baking soda, salt, and cinnamon. Stir in oats and brown sugar. Using your fingers, work butter into mixture until combined; mixture should be crumbly but hold together.
Line an 11-by-17-inch baking pan with parchment paper. Press 5 cups oat mixture into bottom of pan. Spread desired filling over oat mixture. Sprinkle remaining oat mixture over filling. Bake until golden, about 40 minutes. Transfer pan to a wire rack to cool. Cut into 1 1/4-by-2 1/2-inch pieces; makes about 60.