There are multiple methods to update OnePlus phones, but if you're rooted with TWRP installed, one in particular is your best option. It works like a charm and maintains your root status after an update, and you don't even need to hook back up to a computer like you did when you first rooted.
This method is pretty simple. You'll just need to download the OTA update file manually and install it through the TWRP recovery environment you already have, then promptly re-flash TWRP and Magisk. You'll have to do things in a very specific order, but follow along here and you'll be updated in no time.
The easiest way of manually downloading OTA updates on your OnePlus device is to use the Oxygen Updater app from the Play Store. It allows you to download the latest OTA.zip for your device without having to search online, and it usually even has the firmware files before they're uploaded to the OnePlus downloads site.
- Play Store Link: Oxygen Updater (free)
Once the Oxygen Updater app from the link above has been installed, open it to the main screen so you can get started. Scroll through the welcome page by swiping left two times, then on the "Choose your device" page, make sure "OnePlus 7 Pro" is selected. If not, tap the arrow and select the appropriate option as needed.
Swipe the screen again to the "Root access" page, and it explains what you can do with root privileges. Tap "Close," then hit "Grant" on the Superuser request from Magisk to give root access to Oxygen Updater for even more features.
Now on the "Choose your update method" screen, make sure "Full update" is selected before moving on. Again, tap the arrow and select the correct choice as needed. Swipe to the left one last time to the final screen, then hit the "Start App" button at the bottom to get started with downloading the OTA update file.
Now that the setup is complete, you should be sitting on the main screen of the Oxygen Updater app. If there's an update waiting for you, you'll immediately see a message that says "New system update available," along with the specifics below that.
Scroll down to the bottom of the screen and tap the "Download" option to begin downloading the entire system update. Grant the required media permissions when prompted by tapping "Allow," then the download will automatically start.
Note: If you're using a VPN, you might occasionally have to try another server or disable it temporarily for the update to come through correctly. It can otherwise get stuck at "waiting for a network connection" if you don't, so keep that in mind for future updates.
Since you're looking to keep your root access after an OTA update, you might already have Magisk downloaded and ready to go right now. If not, hit up the link below so you can move onto the next part. Make sure to download the latest version of Magisk rather than Magisk Manager.
Now, reboot to TWRP. The fastest way to get into recovery on any OnePlus phone is to enable the Advanced reboot setting in Developer options. Once done, all you have to do is long-press the power button, then tap the "Recovery" option from there.
At this point, your device should be sitting at the TWRP main menu. From here, tap "Install," then you have to locate the OTA file you downloaded earlier. The Oxygen Updater app puts it in a particular place by default, which is outside of all your other folders by itself. This is called the root of your storage, and it makes it pretty simple to locate.
Tap the OTA file once to put it in the install queue, make sure the "Inject TWRP after install" option is selected, then initiate the "Swipe to confirm Flash" slider to begin installing the system update. Since the full system image is around 2+ GB in size, this could take a bit to finish installing. It might seem frozen at the end of step one, but it'll eventually move onto the next step.
Once that finishes, tap the home button to return to the main screen, choose "Reboot," then finally "Recovery."
Now that you have the OTA update and TWRP back onto your system, you'll need Magisk for your root access. After rebooting to recovery, navigate to your "Download" folder and install the Magisk file just like you did the first time by swiping the slider. Once Magisk is done installing, you can tap the "Reboot System" option to boot back into Android like usual.
You now have the latest version of OxygenOS, your TWRP custom recovery is intact, and your root access is still there, too. As a bonus, you'll now receive your system update notifications from the Oxygen Updater app. Doing so will not only make things easier for you as a root user, but you can skip the slow staged rollout process. Immediate updates for all!