Combine oats and buttermilk in a medium bowl; cover and refrigerate overnight.
Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Place buttermilk mixture, flour, and next 6 ingredients (flour through eggs) in a large bowl; beat with a mixer at medium speed until smooth. Fold in blueberries.
Spoon 1/4 cup batter into each of 24 muffin cups coated with cooking spray. Bake at 350° for 15 minutes or until muffins spring back when touched lightly in center. Remove muffins from pans immediately; place on a wire rack.