[OnePlus 6T  Guides, News, & Discussion] the.emilio:
So as I go over the custom roms available in the development section I cannot help but notice how many users state that the rom is causing random reboots. It is usually due to said user flashing the custom rom over a beta OOS version instead of the latest stable build as the OP instructions usually indicate. The reboots are hardly caused by the rom itself. I am putting together this guide to help users go from beta back to stable. This will allow a clean install of the custom rom and usability as it is intended.

This can also be used to just go back to stable from beta if you want to run the stable OOS.

There are 2 ways to do this. One is via TWRP if that is what you prefer or you can also use the MSM tool to flash back to the latest stable. Both ways will wipe and format your internal data. This is due to the mismatch in security patches between beta and stable. So be sure to make a backup of your internal storage if there is stuff there that you need. You have been warned.

NOTE: Highly recommended to use a computer to make things easier. If you do not have a computer available, use the TWRP method listed below to get back to stable OOS. Once OOS boots, you can download your custom rom of choice using your phone and install it.

Here are the links for the latest MSM tool and latest stable zip if you want to use the TWRP method:

Latest MSM Tool

Latest stable OOS to flash in TWRP

TWRP Steps:

Download latest OOS stable from link above

Flash OOS stable build

Flash TWRP installer

Reboot to TWRP

Factory Reset

Format Data type 'yes' when prompted

Reboot to System

Your phone will reboot to OOS. If you want to run OOS stable you are done. Just set up your phone as normal from here.

If you want to run a custom rom, reboot to TWRP, you can transfer your custom rom, gapps, magisk, kernel and twrp installer using a PC. If no PC is available, download your rom and needed files like twrp, magisk etc. using OOS stable first. Then you can reboot to recovery and follow the installation steps provided by your rom developer on their page.

Steps for MSM:

Download the latest MSM tool with link from above. Boot phone to EDL either by powering off the phone then holding Vol + and - with Power or my preferred way via adb by typing adb reboot edl. Plug in phone once in EDL and let MSM do its thing. If you are not sure how to use MSM, the link for the MSM tool outlines the steps. Your phone will reboot to OOS. If you want to run OOS stable you are done. Just set up your phone as normal from here. Note, your bootloader is locked. You will need to do an oem unlock to install twrp and magisk if you want root or to install a custom kernel.

If you want to run a custom rom or root your phone, get to the settings screen.. you can skip setting up your phone so just hit skip on all the prompts for set up. Go to settings, enable developer options. Make sure OEM unlock is selected and USB debugging is selected. Once you do that, do adb reboot bootloader. Then do the following:

fastboot oem unlock

It will reboot the phone. Skip through the set up again and go back to settings. Enable developer options and enabled USB debugging. Then run adb reboot bootloader. In the bootloader, do the following:

fastboot boot <twrp.img> (replace this with the actual file name)

From there, you can transfer your custom rom, gapps, magisk, kernel and twrp installer using a PC. Follow the installation steps provided by your rom developer on their page.