[OnePlus 5 Guides, News, & Discussion] rainmakersgroup:
So i successfully upgraded from Nougat to Oreo. I just wanted to show my method so others can successful do this without worry, complications, etc. I wanted to post my experience because so far the information on how to do it is not clear at all in my opinion. I hope this helps the community.

I have a Oneplus 5, the bootloader was unlocked, i was rooted using magisk v15.3, and i was on Nougat 4.5.15, i was also encrypted, and I am using blu_spark TWRP.

I made a backup of everything I THOUGHT was important either by drag and dropping files(photos, music, etc) to my computer and using titanium backup for my apps(which file i also dragged and dropped to my computer). YOU WILL HAVE TO FORMAT YOUR INTERNAL STORAGE, so making a backup is absolutely necessary if you want to save your data. If you don't care about your data, then there is no reason to back it up, if you don't care about your apps, then there is no reason to use titanium backup(and no reason to drag and drop this file to your computer). What is important data will vary from person to person. I can't tell YOU what is important for YOU. For me, my photos, music, apps, past text messages(i used MyBackUp Pro app to backup text messages and contacts) are important, so i backed them up.

I ALSO MADE A NANDROID BACKUP using TWRP by blu spark which i also copied to my computer. If you don't know what a Nandroid backup using twrp is, you probably aren't rooted, you probably won't even be reading this, and you probably shouldn't be doing this upgrade to O, or any other android tweeks for that matter. I am also going to assume you are familiar with fastboot mode, recovery mode, and the command shell and various fastboot, and adb commands.

My steps and process is as follows. Good Luck.

1) Breathe, take a few short breaths and believe in your ability to do this LOL

2) open a command prompt on your computer, boot into fastboot mode and flash codeworkx TWRP

3) reboot phone and remove lockscreen to just swipe to unlock ( i just did this as a precaution because some people said they had problems if they kept their pin to unlock or pattern to unlock).

4) boot into recovery ( which now should be codeworkx TWRP). The first boot into codeworkx TWRP took a little longer than usual, about 1 minute.

5) wipe -> advanced wipe -> select dalvik and cache -> swipe to wipe

6) format data

7) wipe -> advanced wipe -> select dalvik and cache -> swipe to wipe

8) select reboot -> power off (At this point i had to power off and reboot into recovery as file transfer was not working)

9) power + down to reboot into recovery again

10) file transfer Oreo OS and magisk zip to internal storage

11) install O zip

12) wipe -> advanced wipe -> select dalvik and cache -> swipe to wipe

13) reboot -> system (i did this first before flashing magisk just to make sure OS was working and booted up). This first boot took approx 1 minute

14) now you are presented with the startup and configure welcome screen. Just follow as you would any other welcome screen when you first get a phone.

15) went to "about phone" and confirmed that I am on Android version 8.0.0, OxygenOS version 5.0.1 YAY!!!!

16) I did all the initial steps such as unlocking developer options(within dev options: advanced reboot, enable usb debugging etc.)

17) reboot into recovery(my phone did not reboot into codeworkx TWRP, so i had to flash codeworkx TWRP using fastboot again)

18) now you can either unplug device and boot into recovery by pressing power button + down volume, or by entering "fastboot boot (name of your recovery file)" while in fastboot mode using the command window.

19) wipe -> advanced wipe -> select dalvik and cache -> swipe to wipe

20) flash magisk zip in order to gain root( if you do not want root access you can stop at step 17, or step 16 if you don't want codeworkx TWRP)

21) wipe -> advanced wipe -> select dalvik and cache -> swipe to wipe

22) reboot system and use root checker app to confirm root status

24) restore all the data that you originally saved before starting this process(photos, music, titanium backup, etc.. SIMPLE DRAG AND DROP to internal storage)

24) set up magisk app and confirm you passed the safetynet check

23) CONGRATS!!!! YOU'VE SUCCEEDED