1. Preheat the oven to 350°. In a medium bowl, whisk the all-purpose flour with the whole wheat flour, rolled oats, baking powder, sugar, salt and baking soda. Add the buttermilk and cooled butter and stir with a wooden spoon until the dough is well combined.
2. Turn the dough out onto a lightly floured surface and knead it 10 times. Shape the dough into a 9-inch round loaf and transfer to a baking sheet. Slash an X about 1/4 inch deep in the top of the bread. Bake for about 1 hour, or until the bread is golden and the bottom sounds hollow when lightly tapped. Transfer the soda bread to a wire rack and let cool completely.:
Make Ahead: The bread can be made up to 1 day ahead.