Preheat the oven to 375 degrees F.
Add egg whites, tarter, and vanilla to the mixing bowl. Mix on medium speed until it looks a little foamy.
While the egg whites are mixing, in a small mixing bowl mix oat fiber, flour, and half the Splenda. Mix until fully blended.
After the egg whites look a little foamy slowly add the other half of the Splenda. Then turn the speed up to high. Beat until stiff peaks form.
After you have stiff peaks in the egg whites, carefully fold in the flour mixture a little at a time. You want to fold this in and not stir so you don’t flatten the mixture. We want it to stay nice and airy.
Pour mixture into an ungreased angel food cake pan. Use a spatula to lightly smooth the mixture out a little to make it a little more even around the pan. Place in the center of the oven and bake for 30 minutes at 375F.
Remove from the oven and allow to cool upside down over a large plate.