I have iPhone XS Max and Oticon Opn 1. When using phone calls go from one ear to the other or no sound for a few minutes. I have no problem streaming music. I have no problem if I don't use my hearing when making or receiving calls.

It is possible that the Bluetooth signal is competing with other Bluetooth devices in your environment.  Try setting the "Call audio routing" on "Always Hearing Device" if it is currently on "Automatic."  You can find this setting under Accessibility and Hearing Devices.

If that does not solve the problem, I would forget the pairing of your hearing aids on your iPhone and then reboot the iPhone before pairing the hearing aids again.  

Agree with Dr. Colburn. The other issue could be firmware version. I would recommend contacting your audiologist and ask them to update your firmware. Usually takes 2-5 minutes. I do believe that is the core issue here. Good luck!

The newest IOS 13 has Bluetooth issues.  I found a workaround that usually works for all brands of hearing aids with Bluetooth.

Unpair your aids (Settings, Accessibility, scroll to hearing aids tap on your name and scroll to bottom and "Forget this device"

Back out and delete the app.

Restart phone and open battery doors of the hearing aids, or turn off aids if they are rechargeable

Reinstall app, DO NOT OPEN APP!

Pair your hearing aids by turning them on when you get to the pairing menu under accessibility menu.

once paired, your name should be blue on the hearing devices menu.

now you can follow the instructions to pair again with the software.

The problem may be that the hearing aids have been paired too many times with your iPhone.  Return to the audiologist to put the OPN 1s back on the computer to see how many times they are paired.  If more than 1x, the hearing aids become unstable with smartphone use.  Delete all pairings (in the firmware), then "forget" hearing aid pairings in the iPhone via accessibility tab, then delete the app.  Pair with the iPhone going through Accessibility.  Then add the Oticon ON app.  Do not pair your hearing aids again (on your own).  Sometimes BT connection can be a little slow, but when in the app - you will see it circling until the connection is complete.