Preheat oven to 350°F. Line 12 standard muffin cups with paper liners; set aside.
In a large bowl, whisk together flour, sugar, oats (if using), baking powder, and salt.
In a medium bowl, whisk together buttermilk (or milk), melted butter, oil, eggs, and vanilla extract.
Make a well in the center of the dry ingredients. Add the wet ingredients to the dry ingredients. Stir just until combined, being careful not to over-mix. The batter may be lumpy, which is fine.
Fold in the chocolate chips.
Divide the batter evenly among the prepared muffin cups, filling each cup almost to the top. Sprinkle additional chocolate chips and coarse sugar on top, if desired.
Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs attached.
Transfer muffins to a wire rack to cool. Serve warm or at room temperature.