It depends on why you want to disconnect the OPN from the iPhone.
If it is to connect another Bluetooth device (like a loudspeaker), just set the iPhone MFI section on Automatic selection and the other bt device will take priority over the OPN. No need to disconnect the OPN. Then when you’re done with the bt device, just disconnect it and the iPhone will reroute sound to the OPN again.
If it is to choose the iPhone loudspeaker or the iPhone regular speaker (where you listen by putting the iPhone on your ear) in the phone calling mode, then you are already given an option to select which of the 3 modes you want. No need to turn off bt. But turning off bt will achieve the same thing. Except later on you’d have to enable bt again for the OPN. But some other apps may not give you this option like the phone app, in which case you’d have to just turn off bt like Texas Bob said.