Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large bowl, combine the rolled oats, all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt. Stir until well mixed.
In a separate medium bowl, whisk together the milk, vegetable oil, egg, and vanilla extract until fully combined.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix.
Fold in the semi-sweet chocolate chips evenly throughout the batter.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.