Preheat oven to 325°F (170°C). Spray an 8½×4½-inch loaf pan with cooking spray.
In a large bowl, whisk together flours, salt, and baking soda until well combined. Make a well in center.
In a medium bowl, whisk together buttermilk, molasses, and egg. Add buttermilk mixture to flour mixture. Using one hand like a claw, mix buttermilk mixture into dry ingredients, working from center to outside of bowl, just until combined. Spoon dough into prepared pan, and spread until even. Sprinkle with oats.
Bake until deep golden brown and an instant-read thermometer inserted in center registers 200°F (93°C), 1 hour to 1 hour and 5 minutes. Let cool in pan for 15 minutes. Remove from pan; wrap in a clean tea towel, and let cool on a wire rack. Best served warm.