Preheat oven to 375° F and coat twelve muffin cups (it's best not to use paper baking cups for these)
In a medium-sized bowl, add the flour, oats, flaxseed, pumpkin pie spice, baking powder, and salt. Combine and make a well in the center.
In another medium-sized bowl, add the eggs, pumpkin, sugar, milk, oil, and vanilla.
Add all at once to the flour mix. Stir until just moistened (your batter will be lumpy). Spoon into the prepared muffin cups