Preheat the oven to 350 degrees, and lightly oil and 8 or 9-inch square baking pan.
In a large bowl, stir together the oats, walnuts, ¼ cup of the oat flour, cinnamon, baking powder, and salt. In the baking dish stir together the rhubarb, 1⅓ cups of the blueberries, 2 tablespoons of the oat flour, and the brown sugar. Spread the mixture into an even layer and top with the dry ingredients.
In a medium bowl, whisk together the milk, banana, maple syrup, oil, and vanilla. Pour the wet ingredients over the oat mixture in the baking dish. Carefully bang the pan on the counter a few times before poking the oatmeal with a fork, making sure to get all the way to the bottom. You want the wet ingredients to soak into the oatmeal, but do not stir the mixture. Top the mixture with the reserved ⅓ cup blueberries.
Bake the oatmeal for 35-45 minutes until lightly browned, bubbling, and set on the top. It should still be gooey and moist underneath. Serve the baked oatmeal hot from the oven, or store it in the refrigerator for up to 3 days. Leftovers can be eaten cold or reheated in the microwave in 30 second increments until they are warmed through.