In your main mixing bowl, pour the warm buttermilk directly over your old-fashioned rolled oats. Stir it together quickly and let it sit on your counter for 15 minutes. This creates a creamy oat porridge base that completely hydrates the oats and guarantees your oat flour bread won't turn out dry or crumbly.
While your oats are soaking, stir your warm water, honey, and active dry yeast together in a small measuring cup. Let it sit undisturbed on your counter for 5 to 7 minutes until it develops a thick, frothy foam on top. If it doesn't foam, your yeast might be expired, and you'll want to start over with a fresh packet!
Pour your frothy yeast mixture, the melted butter, and 2 cups of the all-purpose flour directly into your soaked oat bowl. Stir everything together with a heavy wooden spoon until a sticky, shaggy dough forms. Add the fine sea salt and the remaining 1 cup of flour, then turn the dough out onto a lightly floured surface.
Knead the dough firmly by hand for 8 to 10 minutes (or 5 to 6 minutes on medium-low speed if you are using a stand mixer fitted with a dough hook). Because of the real oats, the dough will start out feeling quite tacky. Keep going! As you knead, the gluten develops and it will transform into a smooth, springy, velvet-like ball.
Place your smooth dough into a lightly oiled bowl, turning it over once to coat the top so it doesn't dry out. Cover the bowl with a damp kitchen towel and let it rise in a warm, draft-free spot for 1 hour to 1 hour and 15 minutes, or until it has completely doubled in size.
Gently press the air out of your risen dough and shape it into a tight, even 9-inch log. Place it into a greased 9x5-inch loaf pan. Brush the top of the loaf lightly with a little water, and press a generous handful of raw rolled oats across the top crust. Cover it lightly and let it rise for about 45 minutes until it crowns beautifully over the rim of the pan.
Preheat your oven to 375°F (190°C). Slide your loaf pan onto the center rack and immediately drop the oven temperature down to 350°F (177°C). Bake for 38 to 42 minutes. Your decorative oats on top will turn toasty brown, and the bottom of the loaf should sound completely hollow when tapped out of the pan. Let your oatmeal bread cool fully on a wire rack before slicing!
