Preheat the oven to 375 degrees . In a large bowl, combine the flours, oat bran, baking soda and salt. Add 1 quart buttermilk and stir with a wooden spoon. Stir in more buttermilk (about 1 cup), stopping when the dough holds its shape and is wet and sticky but not runny.
Divide the dough into two balls, place on a floured baking sheet and sprinkle flour on top. Using a sharp knife, score the tops of the loaves with an "X." Bake for 1 hour, then rotate the pan and bake until the loaves are cooked through, about 50 minutes. If the crust is too hard and crunchy, cover the bread with a damp kitchen towel as it cools.
Slice and serve with butter, capers and smoked salmon.