Grease twelve 2-1/2-inch muffin cups or thirty-six 1-3/4-inch muffins cups, or line them with paper bake cups; set aside. (If you don't have enough muffin pans, bake half the batter at a time. Refrigerate remaining batter.)
Stir together flour, rolled oats, baking powder, baking soda, and salt in a bowl. Make a well in the center of the mixture.
Combine egg, milk, brown sugar, oil, and vanilla in another bowl. Add egg mixture all at once to flour mixture. Stir just until moistened (batter should be lumpy.) Fold blueberries into the batter. Spoon the batter into the prepared muffin cups, filling each three-quarters full.
Bake in a 400 degree F oven for 16 to 18 minutes for 2-1/2-inch muffins or 10 to 12 minutes for 1-3/4-inch muffins or until done. Cool in muffin cups on a wire rack for 5 minutes; remove muffins from cups Serve warm. Makes 12 regular-size or 36 small muffins.