For OnePlus 9/9 Pro

Prerequisites

Note
Dirty flashing from OOS12 based version is fine. The required firmware is included in the ROM.
Danger
When coming from OOS to Nameless-AOSP OOS13 based ROM you need to have OOS13 installed.
Danger
For OOS12 based build the required firmware is C.62-C.65 IN BOTH SLOTS! DO NOT SKIP THIS!

Working brains, common sense, lots of coffee

  1. Latest platform-tools

  2. Google USB driver

  3. Developer options enabled: Settings -> About phone -> Tap Build number at the bottom seven (7) times

  4. ADB enabled: Settings -> System -> Developer options -> USB debugging

  5. Required images: dtbo.img, vendor_boot.img, boot.img (put these in the same folder with platform-tools)

  6. Get images from downloads page

  7. ROM zip (put it in the same folder with platform-tools)

  8. Unlocked bootloader

Note
If you are currently on OOS11 it's better to unlock bootloader before updating to OOS12. For unknown reason some devices cannot be unlocked in OOS12.

  1. Reboot your phone to fastboot mode

  2. Open cmd and go to the folder where platform tools are (DO NOT USE POWERSHELL!)

  3. Enter following commands by copypasting one by one:

    fastboot flash dtbo dtbo.img
    fastboot flash vendor_boot vendor_boot.img

  4. Temporarily flash recovery:

    fastboot flash boot boot.img

  5. Reboot your phone to recovery mode, click "Install update" -> "ADB Sideload"

  6. In cmd enter command:

    adb sideload Nameless-AOSP_xxx.zip

  7. Wait until your device shows Step 2/2 and back arrow appears. Click back arrow, then click "Factory reset" -> "Format data/factory reset" -> "Format data"

  8. Click back arrow in recovery, then click "Reboot to system"

Note
Progress on your pc will stop at around 47%. It is normal behaviour.