"Android OS" uses a lot of data in background, how to turn off it?

You need to install a "Root Firewall" in your mobile by which you can control the Internet connectivity of all apps (including system apps). They also offer to block only Data connection and not wifi connection for a specific app.

The app I'm using is "AFWall+" (free).

Note: This type of apps only works on Rooted android phones. Here is the greate article about rooting in android. It is highly recommended for you to know everything about rooting before choosing this option.

If you have decided to go along with AFWall+ and you understand the risk of rooting your mobile device then visit this link to configure your AFwall+ for the first time use.

Further check this answer by @firelord for step by step configuration and screenshots of AFWall+.

If you need further help and have a question related to this answer please comment. I'll be happy to help you.

If you agree then try to use developer options. By using this option when you leave or minimize your app with home button or anything other, your app activity automatically stop. It shows only activity view on task manager but displays activity as not working.

Go to settings -> find Developer option. Tap seven time on developer option and new Developer window will open. Now find "Do not keep activity". If it is not marked, then mark it. By default it will be not marked. Now try to open any app activity and minimize. Then try to restart activity from recent task and see magic. It will not start. It means it stoped.When you want to disable this from working then simple switch off your developer option.

Go to Settings → Data usage → click on the 3 vertical boxes in right top corner → tick the Restrict background data → then remove the tick in Auto sync data.

All other things are also helpful like disabling Auto Sync Background data, etc. Try doing this:

• Go to Settings -> Apps -> All Apps.
• Go to the last app Update Centre and then tap on it.
• After opening it tap on Force close.

I think is is the probable cause of the background data usage by Android OS. And by doing this you may get rid of it.

Note: You need to remember that you will have to do this trick every time you restart the device. If you have a pending System Update, then its icon from the notification tray will disappear. This directly means that your device will no longer search for System Updates until you restart it.

In short, to stop OS using mobile data, force stop Update app.

This is due to Google Backup.Switch Google Backup off and your problem will be solved. Follow these steps:

1. Open Photos app.
2. From options goto Settings.
3. Touch Auto Backup and switch the toggle On or Off.