Preheat the oven to 425°F (220°C).
In a large mixing bowl, combine whole wheat flour, all-purpose flour, oats, baking soda, salt, and brown sugar.
Make a well in the center of the dry ingredients and pour in the Guinness.
Mix until just combined; do not overmix.
Turn the dough onto a floured surface and shape it into a round loaf.
Transfer the shaped loaf to a baking sheet lined with parchment paper.
Score the top of the loaf with a sharp knife.
Bake for 30-35 minutes or until golden brown.
Let the bread cool on a wire rack before slicing.