Preheat the oven to 400°F (200°C). Grease a 9x5-inch loaf pan and line it with parchment paper for easy removal.
Combine whole wheat flour, oats, all-purpose flour, baking soda, salt, and sugar in a large mixing bowl. Mix well.
Gradually pour in the buttermilk, stirring with a wooden spoon until the dough comes together. The dough will be sticky and wet—this is what gives the bread its lovely texture.
Add the butter and mix well.
Transfer the dough to the prepared loaf pan, smoothing out the top with a spatula.
Use a sharp knife to cut a slit down the center of the loaf. This helps it rise evenly while baking. Sprinkle extra rolled oats on top for a rustic finish.
Bake for 45–50 minutes, or until the bread is golden brown and a skewer inserted in the center comes out clean. If the top browns too quickly, loosely cover with foil for the last 10 minutes.
Lay a clean kitchen towel over the bread loaf and let the cool in the pan for at least 10 minutes, then transfer to a wire rack to cool completely before slicing.