Remove Google FRP (Factory reset protection) - Tested on OnePlus 3 running Android O

I bought a used OnePlus 3 for a friend and the previous owner had not logged out before wiping the phone so I was faced with the "This device was reset. To continue, sign in with a Google Account that was previously synced on this device" message.

The solution was to boot the phone, tap the Emergency call button and enter/dial *#812# as illustrated in the following video:

While searching for a solution I also found the following video with a more "complicated" solution that might be of use if the above does not work: