Android 7 or Nougat update for the OnePlus 3t arrived about 2 weeks ago. I was apprehensive about installing it after reading reports of bad battery performance and weird delete problems. Keeping a sharp eye on the online forums, while I check out every nuance of the Marshmallow Oxygen OS.
With no one screaming for help, I figure it is safe to install.
After the upgrade, clear the cache in Storage and clear data for Google Play Service. It ran for a whole day of Fri 12 May with no problems. I accidentally left it unplugged and amazingly still with 40% battery.