Place oats in a food processor; pulse 8 times or until coarsely chopped. Combine chopped oats and 1 cup boiling water in a medium bowl; let stand 10 minutes, stirring occasionally.
Dissolve yeast in 1/4 cup warm water in the bowl of a stand mixer; let stand 5 minutes. Add buttermilk to oat mixture, stirring to combine. Stir in honey and oil. Add the oat mixture to yeast mixture; mix with dough hook attachment until combined. Weigh or lightly spoon 5 ounces all-purpose flour (about 3 cups) and whole-wheat pastry flour in dry measuring cups; level with a knife. Combine flours with salt. Add flour mixture to buttermilk mixture. Mix dough at medium speed 10 minutes or until smooth and elastic, adding remaining all-purpose flour, 1/4 cup at a time, to prevent dough from sticking to sides of bowl. Add walnuts; mix at medium speed just until combined.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, the dough has risen enough.)
Preheat oven to 400°.
Punch dough down; divide in half. Divide each half into 3 equal portions. Working with 1 portion at a time (cover remaining dough to keep from drying), shape each portion into a 14-inch rope. Place 3 ropes lengthwise on a baking sheet coated with cooking spray (do not stretch). Pinch ends together at one end to seal. Braid ropes, and pinch loose ends together to seal. Repeat procedure with remaining dough to form another braid. Cover and let rise 30 minutes or until doubled in size.
Spritz top and sides of loaves lightly with water from a spray bottle. Bake on center rack of oven at 400° for 28 minutes or until deep golden brown. Remove from pan; cool on wire rack.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.