No problem, just set up another port forward in your router, pointing a port to OPI's internal IP on port 443. For example use external port 4443 and forward to OPI on internal port 443.
Then remember that you will need to add the port number in all setups, for instance in the OPI Sync as demo.op-i.me:4443
If you choose this option, you will have to configure OPI using LAN access (https://opi) since the serverside part of the setup does not know about what port you are running OPI on.