Factory reset, will the os revert?
A factory reset deletes all the userdata that has been stored by the user and apps, including your personal files in internal storage.

A factory reset does not make any changes to the system and boot partition (where the underlying OS is stored) or any other non-userdata partition. During a system update, it is mostly the boot and system partition which gets updated and the update doesn't keep the earlier state of Android saved, so a new update overwrites the earlier updates. 

In short, a factory reset wouldn't bring your device back to Android Oreo anymore. If the backup was stored in the cloud, the restore may be triggered during Setup Wizard post the factory reset so you can try that.