Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease lightly.
In a large bowl, whisk together flour, rolled oats, baking powder, baking soda, salt, and cinnamon if using.
In a separate bowl, beat the eggs lightly, then stir in milk, melted butter (or oil), sugar (or syrup), and vanilla until combined.
Pour the wet ingredients into the dry ingredients and mix gently with a spatula until just combined; do not overmix.
Fold in the blueberries and lemon zest carefully to avoid breaking berries and staining the batter.
Spoon batter into the prepared muffin cups, filling each about 3/4 full for a domed top.
Sprinkle a few rolled oats on top of each muffin for texture and appearance.
Bake in the preheated oven for 18–22 minutes, rotating the tray halfway if your oven bakes unevenly.
Test doneness with a toothpick inserted in the center — it should come out clean or with a few moist crumbs.
Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before storing or serving.