Bring 3 cups of water to a boil in a medium saucepan. Add the milk and bring back to a near-boil; remove from heat when steaming and small bubbles begin to form at the edges. Add the molasses, butter and salt; stir until the molasses is dissolved and the butter is melted. Add the oats and let stand until only lukewarm.
Meanwhile, combine the yeast and sugar with 1 cup lukewarm water in your largest mixing bowl; set aside 10 minutes to activate (it will become bubbly and grow).
When the oatmeal mixture is cool, add it to the yeast mixture, beating well with a wooden spoon to combine. Beat in the flour a cup at a time until it's too difficult to stir, then turn out onto the counter and knead, adding more flour as needed, to create a smooth, workable ball of dough. Knead about 10 minutes total. Set aside to rise in a warm spot with a tea towel draped over top, about 1 hour, until doubled in size.
Punch down, then divide into 4 equal pieces, forming each into a ball. Let rise again, about 1 hour.
Meanwhile, preheat the oven to 350°F (175°C) and line a baking sheet with parchment. Dust the dough balls with additional flour for a rustic look, then use a sharp serrated knife to make a few 1/2-inch-deep slashes. Bake 1–2 loaves at a time (refrigerating the remaining dough balls in the meantime to prevent over-proofing) on the lowest rack, about 45 minutes, until well-browned and the bottom sounds hollow when tapped.
Notes
Big batch: This makes 4 loaves — freeze extras, or refrigerate shaped dough balls and bake over a couple of days.Lukewarm matters: Cool the oat mixture to lukewarm before combining with the yeast so you don't kill it.Proof the yeast: The yeast-sugar-water should turn bubbly within 10 minutes; if not, it's dead — start over.Test for doneness: A well-browned loaf that sounds hollow when tapped on the bottom is done.