In a large mixing bowl, stir the yeast into the warm water.
Add the molasses and stir until it's completely dissolved.
Add the all-purpose and whole wheat flours, one cup at a time, mixing well after each addition to form a soft sponge.
Beat the mixture vigorously with a wooden spoon for about 10 minutes until it's smooth and very soft.
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.
Once it's firm enough to handle, turn it out onto a well-floured surface.
Knead the dough for about 20 minutes, adding just enough extra all-purpose flour as needed to prevent it from sticking excessively.
Continue until the dough is smooth and elastic.
Form the dough into a ball, place it in a large, buttered bowl, and turn it once to coat the top with butter.
Cover the bowl with a tea towel and let it rise in a warm place for about 1 hour, or until it has doubled in size.
Gently punch the dough down to release the air.
Divide it into two equal portions and shape each into a loaf.
Place the loaves into two buttered, medium-sized loaf pans.
Cover the pans with tea towels and let them rise for a final 50 minutes.
Meanwhile, preheat your oven to 350°F / 175°C.
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.
Don't be tempted to add too much extra flour during kneading. A slightly tacky dough is ideal for a moist crumb. Over-flouring will result in a dry, dense loaf.
Let the loaves cool completely before slicing.
Cutting into hot bread can release steam and result in a gummy texture.
