Preheat oven to 450 degrees. Line a baking sheet with parchment paper, set aside.
Combine the flours, oats, sea salt, and baking soda together in a large bowl.
Whisk the buttermilk and molasses together in a large measuring cup or pitcher.
Make a well in the center of the flour mixture, then pour in the buttermilk mixture.
Use a fork (or bread whisk) to stir the liquid into the flour mixture, mixing from around the edge into the center.
Lightly knead with floured hands to form a soft dough.
Shape the dough into a circle and place on the parchment lined baking sheet. Press to flatten until dough circle is about 2-inches thick.
Use a sharp knife with a long blade to cut a deep cross into the top of the dough.
Bake in the preheated oven for 15 minutes, then reduce temperature to 400 degrees. Continue cooking for an additional 20 to 25 minutes, until the bottom of the bread sounds hollow when tapped. Transfer to a wire cooling rack and allow to cool slightly. Serve warm with butter.
Notes
You cannot substitute regular milk for the buttermilk. You need the acid from the buttermilk to react with the baking soda, this is what causes the bread to rise. You can however substitute 1.75 cups regular milk mixed with 3.5 tablespoons of white vinegar or lemon juice if needed.