Open X-change uses CalDav as its Calendar system, as such, it can sync across a number of applications so that you can see your up-to-date appointments on your device of choice.
See the below guide for syncing your CalDav (OX) Calendar system to iPhone
1: In OX Webmail, click on the "3 lines" (hamburger) menu in the top left.
2: Click "Calendar"
3: Highlight the calendar you'd like to set up syncing for, select the 3 dots and in the drop-down, select Properties
4: Copy your CalDAV URL.
5: Go to Settings > Apps > Calendar > Calendar Accounts > Add Account.Here, you need to click ‘Add Other Account’ and select CalDav
6: The pop up window to sync to your calendar needs this information:
Hit Next once all fields have been completed.
7: Decide whether to save Reminders & Calendars from your OX Calendar to your iPhone
8: Success! You can now navigate to your iPhone Calendar App to see and manager your OX Calendar.