[OnePlus 6 Questions & Answers] lemon07r:
This worked for me.. Finally. PS - I did in a slightly different order. I installed twrp first, booted into twrp, made sure I was in slot A. If I wasnt, I rebooted to recovery. If I still wasnt I set slot to A manually then rebooted to recovery again. Flashed the stock boot img you linked to boot, then rebooted to recovery slot b (same process I did to get to slot a) then flashed the stock boot img to boot again THEN finally installed magisk. Booted to system, where it hanged a bit, rebooted before getting into android, but booted in just fine after that..

For some reason if you don't flash stock img to slot A (and maybe slot B too, I did this for good measure) before installing magisk everything dies a horrible death and breaks, which can't even be recovered from /boot /system backups..

EDIT: Tried to reboot into recovery, which reverted back to stock for some reason after doing the above steps. So I proceeded to install the bluspark recovery, rebooted into recovery, made sure I was in slot b, made a back up of the boot image, then flashed bluspark kernel. Got stuck at the unlocked bootloader screen. Rebooted into recovery, restored my /boot back. Still stuck when I try to boot into system. Fixed it by booting back into recovery in slot A flashing stock boot img, then into slot b stock boot, then magisk, but also installed bluspark kernel here because I had a feeling I would lose twrp from flashing to stock boot img, and that the issue was caused by twrp (both twrp-3.3.1-16-enchilada-installer-mauronofrio and twrp-3.3.1-x_blu_spark_v9.108_op6 have this issue). Guess what? Turns out I was right. Having twrp breaks everything after you install magisk, so you need to flash the stock boot img before flashing magisk for some reason.. and cant install twrp again unless you dont mind bricking your phone.

EDIT2: F2FS works, just boot into recovery via fastboot (this should be slot b), change /data to f2fs, boot back into fastboot then into recovery again by fastboot (slot b again), flash stock boot img > magisk > bluspark kernel > f2fs_fstab.zip > f2fs_optimize.zip > factory reset/wipe (from twrp before rebooting), done just reboot into system. You of course lose twrp because magisk doesnt like twrp on oos 10.x for some reason, and will need to set everything up again to cause of the wipe. If you dont factory reset/wipe from twrp before booting into system your system will crash and reboot every time you boot into android, then have to do all the steps I listed again.

EDIT3: Never mind, I was wrong, f2fs makes almost all your apps and storage unusable. Not sure if it's cause of the rom (OOS10.3.0) or because we have to flash stock boot.img and get rid of twrp before booting into system. Either way, f2fs has never really been stable or fun to get working so better to stick to ext4.. that few percent performance boost isnt worth it.

EDIT4: This is my last update. After experimenting with different things, bricking my phone, restoring it via fastboot rom, many many times, I've verified the simplest and cleanest way to install magisk.. Do note you wont be able to keep twrp.

Method A - If you already have twrp

Boot recovery into slot a, flash your rom's stock boot.img, then boot into your recovery in slot b (dont boot into slot a recovery because slot a wont have twrp anymore), flash your rom's stock boot.img, magisk and (optional) your kernel of choice (or not) in that order, then you can boot boot into system. That's it.

Method B - If you don't have twrp

Boot into fastboot, and from your cmd/terminal run fastboot flash boot yourtwrp.img (I suggest Mauronofrio's twrp-3.3.1-16-enchilada-Q-mauronofrio.img or newer). Use your volume buttons to select boot into recovery. Make sure your active slot is B, (if it isnt, install twrp (bluspark and Mauronofrio's twrp both work great) by zip, then follow the steps above in method A). Once you've verified you're in slot B flash your rom's stock boot.img, magisk and (optional) your kernel of choice (or not) in that order, then you can boot into system. That's it.

Maybe [MENTION=4712355]mauronofrio[/MENTION] or [MENTION=3873953]eng.stk[/MENTION] can verify why magisk or twrp bricks the system when you install them together on OOS 10.