Preheat the oven to 400°F. Line a baking sheet with parchment paper or lightly dust it with flour.

In a large bowl, whisk together the all-purpose flour, whole wheat flour, oats, baking soda, and salt until well mixed.

Add the cold butter cubes. Using your fingertips or a pastry cutter, rub the butter into the flour mixture until it looks like coarse crumbs.

Make a well in the center of the flour mixture. Pour in the cold buttermilk and stir with a wooden spoon or table knife just until a shaggy dough comes together.

Turn it out onto a lightly floured surface and gently shape into a ball. Do not knead — overworking makes the bread tough.

Place the dough ball on the prepared baking sheet. Flatten it slightly into an 8-inch round. Use a sharp knife to cut a deep cross (about ½-inch deep) across the top of the dough.

Bake for 30–35 minutes, or until the loaf is golden brown and the bottom sounds hollow when tapped. If the loaf isn’t done, flip it over and bake directly on the rack for 5 more minutes.

Transfer to a wire rack. Cover loosely with a clean kitchen towel as it cools to keep the crust tender. Allow it to cool for at least 30 minutes before slicing.