Actually you won’t find the OPN on the list of regular Bluetooth devices as mentioned above. It can only be found under Settings->General ->Accessibility ->MFI Hearing devives.
You can’t really disconnect the OPN anywhere. You can Forget This Device on the OPN if you drill into the OPN connection option, but there’s no option to disconnect. If Bluetooth is on then the OPN is connected, unless the Bluetooth is off, then the OPN is not being streamed to, although it’s still connected to the iPhone via Bluetooth.
The bottom line is that the OPN can’t be treated like same way as a regular Bluetooth device. It’s treated as a special MFI Hearing device connected through the General ->Accessibility->MFI Hearing Device menu.