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Boot into Recovery Mode (Power + Volume Down). Perform a "Factory reset" and "Wipe cache." If that fails, use Method 2 (MSM Tool).
Go to the official OnePlus Community forums. Search for "OnePlus 9 Rollback Package." Download the (approximately 3.5GB). Ensure the file extension ends in .apk (this is a special installer, not a standard OTA). Downgrade Oneplus 9 To Android 11