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Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 10x15-inch baking pan with cooking spray.
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Prepare oat streusel topping: stir oats, flour, brown sugar, and cinnamon together in a large bowl. Cut in butter with a pastry blender until the mixture is crumbly. Set aside.
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For bars. whisk flour, baking powder, baking soda, and salt together in a bowl.
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Whisk together white sugar and oil in a large bowl until well combined. Whisk in milk, eggs, and vanilla extract.
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Stir flour mixture into egg mixture until just combined.
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Gently fold in blueberries.
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Spread batter evenly into the prepared pan.
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Sprinkle streusel topping evenly over batter.
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Bake in the preheated oven until a toothpick inserted into the center comes out clean, 25 to 28 minutes.
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Let bars cool in the pan on a wire rack 20 to 30 minutes before slicing and serving