Mix rolled oats, buttermilk, and molasses in a large bowl until thoroughly combined.
Whisk together whole wheat flour, all-purpose flour, salt, baking soda, and cream of tartar until combined; pour flour mixture into buttermilk mixture and stir just until moistened.
Turn dough out onto a lightly-floured work surface and knead 3 or 4 times until dough holds its shape.
Cut dough in half; pat each half into a 6-inch round loaf.
Use a sharp knife to deeply score each loaf in quarters, cutting 1-inch deep.
Wrap each loaf in plastic wrap and allow to stand at least 12 hours at room temperature to ferment. (Loaves can be refrigerated up to 2 more days after this, if desired.)
Preheat oven to 400 degrees F (200 degrees C).
Place loaves on an ungreased baking sheet.
Bake loaves for 15 minutes and reduce heat to 350 degrees F (175 degrees C). Continue to bake until loaves are browned and make a hollow sound when tapped on the bottom, about 20 more minutes.
Allow breads to cool for 10 minutes before slicing. To serve, slice loaves 3/4-inch thick all across the loaf, then cut slices in half.
