Line a baking sheet with parchment paper or silicone baking mat.
Prep a piping bag with a larger round tip. I use Wilton 2A.
Combine oat flour and powdered sugar, and set aside.
Beat egg whites on medium speed until foamy.
Add cream of tartar, and continue beating until you start to see a pattern from the whisk in the foam.
Slowly and gradually add the sugar while still beating.
Once all the sugar is added, turn the mixer up to high speed and beat until the egg whites form stiff peaks.
Add the oat flour mixture to the egg whites. Stir by running a spatula around the edge of the bowl, and under the egg whites, folding over.
Continue doing this until the batter falls off the spatula, with some breaking, just before the figure-8 stage that most macaron recipes call for.
Add the batter to a piping bag, and pipe onto the lined baking sheets about an inch apart.
Bang the baking sheet on the counter or floor to release any air bubbles.
Let rest for 30 minutes.
Preheat the oven to 300 F.
Bake for 15 minutes. Let cool on the pan completely.