Preheat your oven to 375°F (190°C). Grease your muffin tin or line it with muffin liners for easy removal.
In a large mixing bowl, combine the rolled oats and milk. Let it sit for about 10 minutes to allow the oats to absorb the milk and soften.
Add the light brown sugar, vegetable oil, and eggs to the oat mixture. Whisk until everything is well combined and smooth.
In another bowl, whisk together the all-purpose flour, baking powder, salt, and ground cinnamon.
Gradually add the dry ingredients to the wet mixture. Stir gently until just combined; be careful not to overmix.
Spoon the batter into the prepared muffin tin, filling each cup about two-thirds full.
Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for a few minutes. Then transfer them to a wire rack to cool completely. Enjoy your Oatmeal Muffins warm or at room temperature!