What happens if I uninstall Android OS?

However if by erasing Android you meant deleting the OS, well you can certainly format your /system partition (OR even /dev/block/ partition if you wish to remove the Kernel too), but without replacing it with another OS the phone’s a fancy paperweight.

Can we delete Android OS?

Basically, You cannot delete an OS of Android Smartphone. OS is basic necessity to run hardware to it’s specified programs. Without an OS the smartphone is nothing but just a bunch of hardware which is useless. Yet, You can replace the Stock OS to any other custom rom just in case to get peak performance or any more.