In a large mixing bowl, combine oatmeal, whole wheat flour, brown sugar, salt, and instant yeast. Stir together until well blended.

Gradually pour in lukewarm water and olive oil into the dry mix. Stir until just combined, creating a messy, sticky dough.

Cover the dough with plastic wrap or kitchen towel, letting it rest for about 2 hours until doubled in size.

Lightly flour a clean surface and turn the dough out. Shape the dough into a loaf and place it seam-side down in a greased loaf pan. Cover and let rise for 1 hour.

Preheat your oven to 425°F (220°C). Bake the loaf for 10 minutes, then reduce the temperature to 350°F (175°C) and bake for an additional 40-50 minutes.

Remove from the oven and allow to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely before slicing.