In a microwave safe bowl, combine the water, oats, maple syrup, butter, and salt. Microwave in 30 second bursts until the butter has melted. Allow to cool to 120-130°F and transfer mixture to the bowl of a stand mixer fitted with the dough hook.
Add the whole wheat and all-purpose flours and yeast to a separate bowl; whisk to combine. Add flour-yeast mixture to the mixer bowl. Knead on low (speed 2 or 3) until a smooth, soft, and elastic dough forms, about 10-15 minutes.
Spray a large bowl with non-stick cooking spray and turn dough out into the bowl. Spray the top of the dough with the non-stick spray and cover with plastic wrap. Allow to rise until almost doubled in size, about 1 hour. (See note for overnight rise option.)
Spray two 9x4x4-inch (or 8 ½x4 ½-inch) loaf pans with non-stick cooking spray. Set aside.
Punch down and turn dough out onto a clean surface. Divide the dough in half and shape each piece into a loaf shape about 8 inches long. Place in the prepared loaf pans, seam side down, and cover with plastic wrap. Place in a warm area of the house and allow to proof until they’ve risen slightly above the rim of the loaf pan and when you gently poke the loaves, the indentation partially springs back, about 1½ hours.
Preheat the oven to 350ºF and bake for 35-40 minutes or the internal temperature reads 190ºF.
Tip the loaves out of the pans and allow to cool fully cool on a cooling rack before slicing.