Preheat the oven to 425°F (215°C). Lightly grease a baking sheet.
In a large bowl, stir together whole wheat flour, white flour, baking soda and salt.
Gently mix in the buttermilk until a soft dough is formed. Mix until just combined. The dough should be soft, but not too sticky. Dust hands with flour if needed to handle the dough.
Turn onto a floured work surface and work dough into a round about 1 ½ inches thick.
Place on prepared 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 (200°C) and bake for 30 minutes more or until golden brown. When done, the loaf will sound slightly hollow when tapped on the bottom. Use a towel to hold the loaf and a wooden spoon to tap it so you don’t burn your hands.
Remove from the baking sheet and place on a wire rack to cool.