Factory reset, will the os revert? Hi I just purchased an Android phone that came with Android Oreo. It's replacing a phone that had Android Pie. Shortly after I set up the new Android Oreo phone It upgraded to Android Pie, but because the old phone was on a newer version of the operating system than the new phone when I did the initial set up none of my system backups were restored to the new phone when I was setting it up. 

I'm thinking of doing a recovery mode or a factory reset on the new phone to see if it will bring over my backups and I'm wondering if this phone will revert to Android Oreo or if it will restore to Android Pie which would then restore my backup
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.