In a large mixing bowl, stir the yeast into the warm water.
Add the molasses and stir gently until it's fully dissolved.
Let it sit for 5-10 minutes until foamy to ensure the yeast is active.
Add the all-purpose flour and whole wheat flour to the yeast mixture, one cup at a time, stirring well after each addition.
Once all four cups are incorporated, use a sturdy wooden spoon to beat the mixture vigorously for about 10 minutes. This develops the gluten and creates a smooth, very soft 'sponge'.
Cover the bowl with a clean tea towel and place it in a warm, draft-free spot for 45 to 60 minutes. The sponge should rise significantly and look bubbly and active.
Sprinkle the salt over the risen sponge and gently fold it in. Begin adding the rye flour and rolled oats, a little at a time, folding them into the dough. The dough will become quite thick and shaggy.
Once the dough is cohesive enough to handle, turn it out onto a well-floured surface. Begin to knead, adding just enough extra all-purpose flour as needed to prevent sticking.
Knead for about 15-20 minutes, until the dough is smooth and elastic.
Form the dough into a ball.
Place it in a large, lightly buttered or oiled bowl, turning it over once to coat the top.
Cover with a tea towel and let it rise in a warm place for about 1 hour, or until doubled in size.
Gently punch down the risen dough to release the air.
Divide it into two equal portions.
Shape each portion into a loaf and place them into two medium-sized, buttered loaf pans.
Cover the pans with tea towels and let the loaves rise for a final 50 minutes. They should rise just above the rim of the pans.
While they rise, preheat your oven to 350°F or 175°C (350°F).
Bake for 1 hour, or until the loaves are deep golden brown and sound hollow when tapped on the bottom. Immediately remove the loaves from the pans and let them cool completely on a wire rack before slicing. This prevents the interior from becoming gummy.