Preheat the oven to 425°F. Lightly grease a baking sheet.
In a large bowl, whisk together whole wheat flour, white flour, baking soda and salt.
Gently mix in the buttermilk until a soft dough is formed. Knead very lightly. The dough should be soft, but not too sticky. Dust hands with flour if needed.
Turn onto a floured work surface and work dough into a round about 1 ½ inches thick.
Place on a baking sheet.
Score the loaf with an 'X' about ½ inch deep.
Glaze the bread with the leftover bit of buttermilk and dust the top with rolled oats.
Bake for 15 minutes, then turn down the oven to 400°F and bake for 30 minutes more or until golden brown. When done, the loaf will sound slightly hollow when tapped with a wooden spoon.
Remove from the baking sheet and place on a wire rack to cool.