This article only applies to users running custom / TWRP recovery. Don't know what this is? You're probably fine and can continue to update your device using the installation guide present in the app.
UPDATE #1 (March 10th, 2018): Official Oreo-compatible versions have been released for the 5 and 5T. It is at your own risk to install these, but forum reports indicate these are indeed compatible (after many previous versions turned out not to be compatible).
This article applies to: OnePlus 3, 3T, 5 and 5T. Older OnePlus devices aren't capable of running Oreo and are therefore not affected by Oreo compatibility issues
As you might already know, there are some issues with TWRP recovery once you've updated your device to Android Oreo. From what it seems, OnePlus has changed the encryption of the phone, causing TWRP not to be able to read its contents.
Image credit: Espcializados
That is a major issue, because you can't flash, backup or restore anything to your device. Basically, the only options left are a full wipe of your phone, or reverting to stock recovery and losing the ability to flash custom software.
To make matters worse, wiping the phone using TWRP might cause bootloop issues. This happens, because OnePlus states that its firmware only supports devices with a locked bootloader (and thus requires prescence of the new encryption method which older versions of TWRP don't support).
Update your TWRP to the latest version. That is: any version equal to, or higher than 3.2.1-1.
Installation takes place using fastboot mode on a computer - not in the Oxygen Updater app. It's the same process as when you first flashed TWRP onto your device.
Installing using fastboot:
You'll probably already be familiar with the install process of TWRP, but in case you're not: Here's a short summary on how to install it.
- Download fastboot using this awesome guide from Lifehacker: Fastboot install guide for all Operating Systems
- Boot your device into fastboot mode: For OnePlus 3 / 3T, all you have to do is power your device off, then start it up while pressing both the power and volume down buttons at the same time. For OnePlus 5 and OnePlus 5T, you also have to turn your phone off, but this time boot to recovery mode using the Power and Volume Down buttons. When in Recovery Mode, tap on your language, then on Advanced and then on Reboot to Fastboot (twice). Your phone will now enter fastboot mode.
- Make sure your bootloader is unlocked. Otherwise, you can't install any custom software on your device. Bootloader not unlocked? Connect your phone to your computer, open a Command Prompt and run fastboot oem unlock. Note: This will erase *all* files from your phone. Please make a backup before proceeding.
- When you are going to install TWRP, boot into fastboot mode (again) and enter the following command: fastboot flash recovery [your-twrp-filename.img]. Don't forget to replace the blockquotes and your-twrp-filename.img with the filename of the downloaded TWRP version for your device.
*Note: ONLY install a TWRP version made for YOUR device. TWRP builds are device-specific software and will NOT work on other devices!*
OnePlus 3 / 3T:
The latest official version is not yet confirmed to be compatible with Oreo. Therefore, you'll have to install a custom version of TWRP (i.e. one that is developed by someone else than the official maintainer). In this case , there is a universal TWRP version for both OnePlus 3 and OnePlus 3T available on XDA-developers, courtesy of XDA user @akhilnarang.
To obtain it, go to this link: https://forum.xda-developers.com/oneplus-3/how-to/twrp-root-android-8-0-oreo-t3689032. Even though this link address mentioned OnePlus 3, the developer states it is also compatible with OnePlus 3T.
You can download the latest official version from: https://dl.twrp.me/cheeseburger/twrp-3.2.1-1-cheeseburger.img.html
You can download the latest official version from: https://dl.twrp.me/dumpling/twrp-3.2.1-2-dumpling.img.html
Wrapping it up:
Installing a newer version of TWRP is required to be able to install updates on Android Oreo and later. From what it seems, all versions mentioned here are stable enough to be used on your main device. But note: no single piece of software is perfect - thery may always be a bug in your version of TWRP. If that happens, please report it to the TWRP team (OnePlus 5 / 5T users), or to the developer who made it (OnePlus 3 and 3T users)
Icon credit: Team Win, founders of TWRP recovery