The OPN only connects to the ON app for control, but it’s not the ON app that’s dropping connection. You don’t even need the ON app all. The primary connection that can let you change programs and volume is actually the MFI functionality from the iPhone. Once the MFI pairing to the OPN is set up, just triple click on your home button and the MFI control window will show up that lets you change programs and control volume just like the ON app does.
Oticon makes the ON app primarily to provide Oticon-specific functionality to users (beside program change and volume control) like the HearingFitness thing, IFTTT, instruction for use, tinnitus, etc. But if you only care to use the program change and volume control, I personally think it’s better to use the MFI menu provided by the iPhone for that, instead of having to connect to the OPN to do that.
Just make sure you’re on Firmware 6 with the OPN and iOS 12.0.1 or latest on the iPhone. The connection improves a lot with this latest combination. I haven’t lost any connection between my OPN 1 and the iPhone while talking on the phone with this combination. On very long driving trip when I listen to music for hours, once in a while there’d be a dropped connection on one ear but in a few seconds, it reconnects by itself.