Heat the oven to 375°F with a center rack. Lightly spray a standard 12-cup muffin pan or spray-lined paper cups.
Blend oats in two batches for 30 to 45 seconds each, stopping and stirring safely, until powdery with no intact flakes.
Whisk the oat flour with baking powder, baking soda, cinnamon and salt, breaking up all leavener clumps.
Whisk eggs, Greek yogurt, maple syrup, oil, vanilla and optional lemon zest until smooth.
Fold wet into dry just until no flour pockets remain. Do not blend the complete batter at high speed.
Rest batter at room temperature for 10 minutes. It should fall in a thick ribbon and a groove should close slowly.
Reserve optional top berries, then fold the rest into the batter with 5 to 7 broad strokes. Keep frozen berries frozen.
Divide among 12 cups, filling about three-quarters full. Add reserved berries and optional rolled oats.
Bake 18 to 23 minutes with fresh berries or 20 to 27 minutes with frozen, until tops spring back and a crumb-area toothpick has no wet batter.
Cool in the pan 10 minutes, then transfer muffins to a rack and cool at least 30 minutes before judging the crumb.
