This is the safest method. OnePlus hides these rollback APKs on its community forums, but they expire after a few months. Here is the way to find a working one.
What is your exact (found in Settings > About Device)? downgrade oneplus 9 to android 11 exclusive
Install the "Local Update" APK you downloaded. If it's labeled as a .zip , rename it to .apk first. Initiate Rollback: Open the newly installed Local Update app. Tap the gear icon in the top-right corner. Select the Android 11 rollback package from the list. This is the safest method
Connect your OnePlus 9 to a computer using a USB cable. Select File Transfer mode on your phone, and copy your DCIM , Pictures , Downloads , and Documents folders directly to your computer's hard drive. What is your exact (found in Settings > About Device)
fastboot delete-logical-partition system_a fastboot delete-logical-partition system_b fastboot delete-logical-partition product_a fastboot delete-logical-partition product_b fastboot delete-logical-partition vendor_a fastboot delete-logical-partition vendor_b
to Android 11 (OxygenOS 11) is a common move for enthusiasts . However, this process is high-stakes; it will , so a full external backup is mandatory before you begin. Essential Pre-Flight Checklist