[OnePlus 6 Guides, News, & Discussion] cc_rock:
EDIT: After a few reboots, TWRP stopped working.. now if I try to enter recovery, I get into fastboot instead. But I can still boot the system fine and I still have Magisk. Sorry.. looks like TWRP on this version is still a no-go for now..

I leave the guide here to keep the discussion open, though.. and after the steps some more info on what exactly worked for me..

Hi all, the following steps worked for me (not really, see edit above), but as usual please proceed at your own risk, backup everything before, and be prepared to reinstall everything from scratch if something goes wrong. I am not responsible of anything that happens to your devices when following this guide.

Prerequisites:

- OnePlus 6 (might work also for 6t, but it's untested)

- Oxygen OS 9 open beta 25 (Might work also with the stable version, but again, untested)

- Enable the "advanced reboot" option in developer options.

- latest blu_spark TWRP installed (3.3.1 v9.101)

- if you have Magisk, make sure it's version 20.0 or later (previous versions don't support Android 10)

- if you have Magisk modules, it's highly recommended to disable or uninstall them before proceeding, as some of them might not be compatible with Android 10 and eventually cause a bootloop.

- A computer that is able to run fastboot commands and that you can connect your phone to.

Now, the steps:

- Reboot into TWRP, open the "reboot" menu and take a note of your current active slot (can be a or b)

- Go back to Oxygen OS and install Android 10 open beta 1 through system updates. At the end of the installation, DO NOT PRESS "REBOOT".

- Press the power button and from the advanced reboot menu, reboot into bootloader.

- Connect the phone to the pc.

- If your active slot, noted in the first step, was "a", type "fastboot --set-active=a" . If it was "b", use the same command but replace the final letter with "b".

- On the phone, use the volume buttons to select "recovery mode", and press the power button to confirm.

- You should now be into TWRP: install TWRP again using its installation zip file, DO NOT REBOOT.

- Go to the reboot menu, and change the active slot to the other one (depending on your currently active one).

- Still in TWRP, reboot to recovery.. you should land in TWRP again.

- If you had Magisk, or if you want it, flash it.

- Reboot to system.

If everything goes well, you should now be on Oxygen OS 10 open beta 1, with TWRP installed!

Note: the current version of TWRP does not support flashing the new OnePlus open betas, so when a new open beta is released and you want to update, you will have to go through this process again (until a new compatible TWRP is released).

EDIT 2: Just to give some more information in case someone else wants to try, here's what I did AFTER the steps below:

- Rebooted into OOS open beta 1, just to check that it was booting.. worked.

- Rebooted into TWRP and did a complete wipe, including the internal storage (I wanted a clean install of OB1)

- Rebooted into OOS and did the phone setup (Google account, ecc) including setting up a lockscreen pattern, but no figerprints yet.

- Rebooted again into TWRP to install Magisk, and everything worked, TWRP also asked for my pattern and decrypted data correctly

- After that I didn't reboot anymore for a while, and completed the setup with apps, fingerprints, etc.. and also installed a magisk module (quickswitch), but without using recovery.

- The next time I tried to reboot into recovery, I wasn't able anymore.. I was getting into fastboot instead, but the system can still boot just fine.