In a large bowl, rub the orange zest into the sugar to release its oils, about 1 minute. Then whisk in the oat flour, baking powder, and salt. Toss the cranberries into the flour mixture, stirring to lightly coat them with flour.
In a separate bowl, whisk together the egg, oil, and milk. Gently fold the wet ingredients into the dry ingredients.
Preheat the oven to 400°F and rest the batter while the oven heats. Lightly grease a 12-cup muffin pan, then use a ¼-cup measuring cup to fill each muffin cup about ¾ full.
Bake the muffins for 20-25 minutes (check at 20 minutes), until a tester inserted into the center of one comes out clean. Remove the muffins from the oven and cool in the pan on a rack for 5 minutes, then turn them out onto a rack to cool completely.
Notes
Add orange zest to the sugar first. For the most fragrant and flavorful muffins, zest the orange into the sugar in a bowl then rub it in using your fingers. This releases the citrus oil in the peel and will give the muffins a much more orangey flavor.
Rest the batter. Oat flour needs a bit of time to hydrate, so let the batter rest for up to 20 minutes or at least while the oven preheats so that the flour properly absorbs the liquid.
Bake at a high temperature. This recipe calls for baking the muffin batter at 400F - it's not a typo! The high oven temp will help the muffins achieve a nice rise with beautifully domed tops.
To serve: Softened salted or unsalted butter is a must, and you can slightly rewarm the muffins in the oven, toaster oven, or microwave so that the butter melts slightly ... the best!