Mix oats and buttermilk in a large bowl; allow to rest for 15 minutes.

Preheat the oven to 400 degrees F (200 degrees C). Grease 24 muffin cups or line with paper liners.

Combine eggs, brown sugar, and butter in a small bowl; stir into oat mixture. Mix flour, baking powder, baking soda, cinnamon, and salt in another bowl; stir into oat mixture until just moistened. Fill prepared muffin cups about 3/4 full.

Bake in the preheated oven until a toothpick inserted into the center of a muffin comes out clean, 15 to 18 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.