[duepointe]

I read that the Oreo up(or down...)grade installed itself automatically for some people. Is there a way to avoid it ? Thanks.

Not sure if it works on your end.
Anyone already tried blocking the OTA domain to prevent the update? 
I just tried setting up an openDNS account and blocked
android.googleapis.com
and it somehow prevented me to download the OTA file


if your router supports custom scripts and configs like AsusWRT by Merlin you can try this approach.
I created a host file to block the domain.
/jffs/configs/hosts.add
127.0.0.1 android.googleapis.com
and it also blocked me from downloading the OTA file.