[OnePlus 6 Questions & Answers] Be4st187:
What some guides miss out is that you have to install everything twice. Once on slot A and once on slot B and after each install, verify what slot you are on before reboot and then check what slot you are on after reboot. I ran into issues flashing Havoc OS onto my phone too and figured it out in the end after so many times that I was in fact installing stuff twice on the same slot because it never switched over. I'll run you through the order I did things so hopefully you can follow.

-Use MSM Tool to revert everything back to stock and once the phone is booted, update to 9.0.9 either by downloading the update through the phone or by local upgrade if you have the file to hand. (remember to only have the OOS file present as any other OS in zip form crashes the local updater). You don't need to do this if you are already rocking stock Oxygen OS.

-Once the phone has restarted after the upgrade, let the post update procedure finish before continuing.

-Enter developer mode and enable OEM unlocking, Advanced Restart and USB Debugging. From here, reboot to bootloader.

-Plug your phone into the PC now and assuming you have fastboot setup ready on your PC, type 'fastboot oem unlock' and use the volume keys to select the option to unlock and press the power button to select.

-After the phone has completed it's wipe cycle, re-enable Advanced Restart and USB Debugging. The OEM unlock option should be greyed out as it's already been enabled. Make sure you don't add a pin code or password just yet as you will enable encryption which will make the next steps harder than it should be.

-Boot back to bootloader and type 'fastboot boot twrp.img' (or whatever you named it. Make sure that the image file is in the same folder you initiated cmd or powershell)

-Once booted into TWRP, flash blu_spark TWRP as that supports encryption apparently and is more stable than any other. To make sure that the script executed on both slots, hit reboot and check what slot you are on and tap the button for the other slot. Now reboot back to recovery, check you are on the other slot and flash it again.

-Now flash your 9.0.9 OOS zip followed by TWRP once on slot A and once on slot B while making sure you are on the other slot when doing it a second time. Remember that you have to re-flash TWRP after any flashable file that alters the boot partition before you reboot or you will undo all your work. Just make it habit that you re-flash it every time and you should be good.

-Once OOS and TWRP have been flashed on BOTH slots, reboot back to recovery and do the whole process again with your custom ROM (Remembering to flash TWRP before you reboot and checking which slot you are one before and after each flash).

-Once that has completed, reboot back to recovery and flash Magisk on each slot and GApps or MicroG if your ROM supports it. You don't need to re-flash TWRP after this but I do anyway to keep the habit lol.

-Entirely up to you, but before you reboot to your fresh new OS, you can wipe internal storage ONLY if you want all traces of your previous OS gone, but this will also remove personal files etc. I generally just head into TWRP file manager and delete the Android folder and keep the rest.

If you run into issues like crashes on your new OS, reboot to recovery and do a factory reset, but you will also need to re-flash Magisk again. GApps installs itself to system, so you won't need to re-flash that. MicroG does not and will need setting up again. One thing that took me a while to realise is that if you try rebooting while your USB is plugged into your PC, you will sometimes enter download mode, so only keep it plugged in when you need it for MSM Tool or to transfer files across.

I'm pretty sure I put everything down the way I did it and didn't miss anything, but it is late and I am half asleep. Will check back in the morning and double check with a fresh brain.