Preheat oven to 400°F. Grease a 9- to 10-inch cast iron skillet; set aside. If you don’t have a cast iron skillet, you can use a large baking sheet greased or lined with parchment paper.
In a large bowl, whisk together flour, oats, sugar, baking powder, baking soda, and salt. In a small bowl, whisk together buttermilk, molasses, and melted butter. Make a well in the center of the dry ingredients. Stir the buttermilk mixture into the flour mixture; stir with a fork or wooden spoon just until combined. Add extra buttermilk, one tablespoon at a time, if needed, to bring the dough together. It will be shaggy and slightly crumbly – that’s fine.
Turn the dough onto a lightly floured surface. Gently knead the dough 3-4 times so that a soft dough forms, and then shape into a round loaf. Place in the prepared pan. Use a sharp knife to score a deep “X” across the top of the loaf. Brush with a little bit of buttermilk; sprinkle with about 1-2 tablespoons of oats and/or seeds.
Bake for 30 minutes. Reduce the oven temperature to 350°F. Bake until golden brown, about 15-18 more minutes, tenting loosely with foil if necessary to prevent excessive browning.
Transfer to a wire rack and let cool before slicing and serving.