I'm having the same problem. I already did a factory reset and monitored the "Android OS" data usage with a fresh factory reset device but there was no improvement. Even if I disable the background data, that Android OS is still consuming background data. I only got a 1GB data monthly so 232MB taken by that Android OS is very frustrating.
Edit: Just fyi, I use my phone data for Clash of Clan, IM apps and email. I only limit myself on those apps due to my very limited data plan.