An Oaty Biscuit, is the act of wearing a condom during sexy time with your partner and crumbling a biscuit over the condom. You then proceed to go down on said partner and make "Sweet Love" or "Fucking" in your preferred position.
When a man has sex with a girl who is lubed up with another man’s man fat, his penis is now referred to as an 'oaty dunker', because he has stirred another man’s porridge…
Big Dave, “Oh man, I was with Katie last night, it was great! What a filth bag”
Little Jonny, “No way man, me too! I went in straight after you, then afterwards she cleaned my oaty dunker with her mouth. Total filth!”