Heat oven to 350ºF.
Arrange oats in a single layer on a sheet pan. Toast oats in oven until fragrant, 6 minutes; transfer to food processor. Turn oven off and close door to keep warm.
Meanwhile, whisk together buttermilk, eggs, butter, maple syrup, and vanilla; set aside.
Process oats until finely ground into a flour; transfer to a bowl. Add all-purpose flour, baking powder, salt, baking soda, nutmeg, cinnamon, and cardamom and whisk to combine; make a well in the dry ingredients.
Whisk buttermilk mixture into flour mixture just until combined.
Heat a nonstick skillet or griddle over medium-low heat for 8 minutes. Lightly coat a skillet with butter or oil, then test temperature of pan with a small drop of batter (it should sizzle).
Spoon ¼ cup batter onto skillet and cook until golden brown and bubbles form and burst on top of pancakes, about 2 minutes. Flip pancakes then cook until golden brown, about 2 minutes more.
Transfer pancakes to an oven-proof plate or platter and keep warm in oven. Repeat with remaining batter.