I've tried oatmeal masks. I found them to not improve my acne, and they're also quite messy. The best mask I've found is honey/yogurt, but even then it doesn't really improve acne per se, more skin texture.

Oatmeal never helped me, either. I never used it as a mask but rather as a face wash. It made my skin feel very smooth and temporarily calmed the redness, but overall didn't help the acne.