Step 1Make the streusel: In a medium bowl, stir the flour, oats, sugar, baking powder, cinnamon and salt together to combine. Add the cubed butter and toss until each cube is fully coated in flour.Step 2Cut the butter into the flour mixture with your hands or a pastry cutter until the mixture forms large clumps. Refrigerate while you make the batter.Step 3Heat the oven to 400 degrees. Lightly grease a 9-by-5-inch loaf pan with nonstick spray.Step 4Make the cake: In a medium bowl, toss the blueberries with 1/4 cup/30 grams all-purpose flour. In another medium bowl, whisk the remaining 2 cups/260 grams flour with the baking powder, baking soda and salt to combine.Step 5In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, oil, granulated sugar and brown sugar until light and fluffy, 4 to 5 minutes. Add the egg, and mix well to combine. Scrape down the sides of the bowl, then add the vanilla and mix to combine.Step 6Add half of the flour mixture, then mix on low speed to combine. With the mixer still running, add the buttermilk in a slow, steady stream and mix until fully combined. Add the remaining flour mixture and mix until just combined.Step 7Remove the bowl from the mixer and, using a silicone spatula, fold in the blueberries (and any remaining flour in the bowl with them) by hand until they are evenly distributed throughout the batter.Step 8Pour half of the batter into the prepared pan and spread it into an even layer. Crumble half of the streusel on top in an even layer. Pour the remaining batter into the loaf pan, spread into an even layer and crumble the remaining streusel on top. The loaf pan will be just over 3/4 full. Place it on a parchment-lined baking sheet.Step 9Transfer the loaf to the oven and bake for 20 to 25 minutes, until the bread has risen about 1/2 inch above the edge of the loaf pan. Lower the temperature to 325 degrees and bake until a toothpick inserted into the center comes out clean, 45 to 55 minutes, tenting the loaf with foil if it’s browning too quickly.Step 10Set the cake on a wire rack and let cool in the pan for 15 to 20 minutes, then run an offset spatula around the outside edge and gently unmold the cake onto a wire rack. Cool completely before slicing and serving.