Since finding anything on these sort of procedures is very hard, I decided to share my experience. I am not responsible for any damage to any device caused by doing the following. The Oxygen OS version I used this on was 10.0.10, other version may require extra steps.

Oxygen OS is one of the nicest skins over Android. It also supports several device-related function in Oneplus devices which many of custom roms fail to utilize due to closed source of the code that Oneplus uses to control the hardware. I usually use Lineage OS on my phone. However, sometimes the official Lineage OS is not available for (especially new) devices. Besides, as mentioned before, if one wants to take full control of their phone camera, they are pretty much stuck with the Oxygen OS (also similar for Xiaomi devices with MIUI and Samsung devices, although ANXCamera can be installed on custom roms for Xiaomi devices and provide close-to-factory-app experience for camera). Long ago and on my Oneplus 3 and Oneplus 5 devices, it was very straight forward to remove all bloatware and most of system/Google apps from Oxygen OS. Recently though, I found that it is very difficult to achieve this with normal procedures I would take on any other device which is the following:

• unlock the bootloader
• install magisk
• install debloater and microG revived magisk modules
• uninstall all unnecessary bloatware from /system/apps, /system/priv-apps and /product folders using debloater

I found that debloater is not able to remove system apps from Oxygen OS anymore. My alternative approach which was using Titanium backup failed, too. Manually deleting the folders from TWRP also led to Oxygen OS not booting and requiring a fresh install, forcing re-encryption of the internal storage, deleting all data. After a long tinkering time, losing data several times, I was about to send back the phone I bought to replace it with a more tinkerable OS (read a phone which would allow me to run its OS without unwanted system and Google apps). I suddenly discovered that the App manager app that I use to limit apps access to my device hardware has the ability to uninstall system apps without causing a bootloop and/or any other issue with uninstallation. So now the procedure is:

• unlock the bootloader (fastboot oem unlock in fastboot mode - WARNING: this deletes all your data)
• install magisk (go for canary channel if you want banking apps to work on your device, more info later)
• after booting to Oxygen OS, install App manager and uninstall Google/system apps
• install microG revived magisk module and restart