Put the oats into a large bowl and add the milk, hot water, honey, and butter. Stir, then set aside to cool.
Pour the warm water into a small bowl and sprinkle with the yeast. Stir and set aside for 5 minutes.
To the oats mixture, add the dissolved yeast, buttermilk, whole wheat flour, egg yolk, salt, and 1 cup of unbleached flour. Beat with a wooden spoon for 100 strokes. Set this “sponge” aside for 5 minutes. Add the remaining unbleached flour, 1⁄3 to 1⁄2 cup at a time, stirring after each addition, until you have a firm dough.
Turn the dough out onto a floured work surface and knead for 9 to 10 minutes, or until smooth, supple, and elastic. Dust your hands and the surface with flour, as needed, to keep the dough from sticking.
Oil a large bowl, add the dough, and rotate it to coat the entire surface. Cover the bowl with plastic wrap and set in a warm, draft-free spot until doubled in bulk, 1 to 1-1⁄2 hours.
Butter two 8.5x4.5-inch loaf pans and dust them with oats.
Punch down the dough and turn it out onto the floured work surface. Knead for 2 minutes. Divide the dough in half, shape into balls, and let rest for 5 minutes. Shape into loaves and place in the prepared pans. Cover loosely with plastic wrap and set aside to rise in a warm, draft-free spot until doubled in bulk.
Preheat the oven to 400°F.
Uncover and place the loaves on the lower oven rack, evenly spaced. Immediately lower the heat to 375°F. Bake for 45 minutes, or until the tops are golden.
Turn the loaves out of the pans and onto a cooling rack.