Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.

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

Gradually pour in buttermilk into the dry mixture, stirring gently until just incorporated. If the dough feels too dry, add an additional 1/4 cup of buttermilk.

Turn the dough out onto the baking sheet, shape into a round ball, and cut an 'X' about 1/2 inch deep in the center.

Bake in the preheated oven for approximately 30 minutes until golden brown and hollow-sounding when tapped on the bottom.

Remove the bread from the oven and transfer to a wire rack to cool completely before slicing.