[OnePlus 8T Guides, News, & Discussion] JimDandy68:
I'm posting here to detail how I solved an OTA update issue with my OnePlus 8T. For disclosure, I purchased this phone used, like new from someone who themselves got it open-box from a reputable retailer. So I don't know the full history of the phone, only that when I first booted it, it had clearly been factory reset and has been running flawlessly.

After I got the phone, I started looking into options for rooting. I unlocked the bootloader, and then, seeing that there was an OTA update for 11.0.9.9.KB05AA, prepared to try to root. I ultimately downloaded a Magisk-pre-patched boot image, thanks to Xi Jing Pooh (whoever you are), linked in these forum.

So after updated to the 11.0.9.9 OTA, I flashed that image, gained root, and again, didn't have any issues.

However, when the 11.0.10.10.KB05AA rolled out, I ran into problems. The OTA update kept failing with the message "Couldn't update, Installation problem."

I had prepared to update by following instructions on the forum; going into Magisk and restoring the stock boot image (an option when you choose "Uninstall Magisk"). I think it turned out I may flashed the pre-rooted boot image before ever booting into 11.0.9.9, so that might have been a problem. So I then used fastboot to flash the stock 11.0.9.9 boot image. I even did this to both a and b slots (maybe not necessary, I'm not sure).

While I did successfully unroot, my attempts to OTA update still kept failing. I then tried to manually update using a local copy of the OTA zip and got the same error.

After discussing with and getting good advice from @BillGoss and @g96818, I decided I would need to use the MSM tool to fully restore my phone to a previous OOS version and then incrementally update. @BillGoss also pointedly recommended I should do a full, manual update to 11.0 8.13 before letting OTA updates install, to ensure my file system had no inconsistencies.

Before I did this, I decided to try to capture a logcat while once again trying to manually update using my local zip of the 11.0.10.10 OTA. I started the update then immediately issued the logcat command in adb shell. As soon as the OTA install failed, I stopped logcat, then scrolled back to find the source of the error. Turned out that my dsp partition had been modified at some point and its SHA did not match that expected by the update.

@BillGoss pointed out to me later that 1) it's likely that the 11.0.9.9 OTA didn't touch the dsp partition, and that's why I had no problem with that update, and 2) I could have tried to dump the dsp parition from the full OTA for 11.0.8.13 and fix the issue without reverting to using the MSM tool.

But, I went ahead and followed detailed instructions to restore the phone to 11.0.7.9 (the latest MSM tool version). Per Bill's advice, after getting the phone setup again, I ignored prompts to download the OTA for 11.0.10.10 and manually ran the full OTA for 11.0.8.13. Finally, I OTA upgraded to 11.0.10.10, this time with no issues at all.

I forgot to unlock my bootloader during this process, so ended up doing that and setting up the phone twice. Then I rooted again, this time using the alpha TWRP that can be found here and installing the Magisk zip, which itself patches and installs the rooted boot image. This seems to have worked fine, and I've tested that I can also easily restore the stock boot before attempting to install the next OTA that comes out.