Prep the pan and oven: Heat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly coat with nonstick spray.
Mix dry ingredients: In a large bowl, whisk flour, oats, sugar, baking powder, baking soda, and salt.
Break up any lumps.
Whisk wet ingredients: In a separate bowl or large measuring cup, whisk egg, Greek yogurt, orange juice, orange zest, oil, and vanilla until smooth.
Combine gently: Pour wet ingredients into dry. Stir with a spatula just until the flour disappears. The batter will be thick—don’t overmix.
Fold in cranberries: Add chopped cranberries and gently fold.
If using frozen cranberries, stir them in straight from the freezer.
Portion: Divide batter evenly among the 12 muffin cups. If you like a hint of crunch, sprinkle a tiny pinch of coarse sugar on top of each (optional).
Bake: Bake 14–17 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs. Do not overbake.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack.
They continue to set as they cool.