Preheat oven to 375 degrees F (190 degrees C). Grease 12 muffin cups or line with paper muffin liners.

Whisk flour, sugar, oats, baking powder, and salt together in a bowl.

Measure out 1 cup of reserved mandarin orange liquid; add liquid and vegetable oil to flour mixture and stir until batter is well-combined. Fold oranges and chocolate chips into batter. Spoon batter into prepared muffin cups.

Bake in the preheated oven until golden brown, about 25 minutes.