To downgrade your to Android 11 (OxygenOS 11), you can follow two primary methods. Both methods will completely wipe all data
Once you've reached the Android 11 setup screen, you can check for minor OTA updates in to get the latest stable version of OxygenOS 11. downgrade oneplus 9 to android 11 exclusive
Exclusive flash of modem , dsp , aop , and tz partitions from the exact Android 11 build matching your region. To downgrade your to Android 11 (OxygenOS 11),