If you sync with a WebDAV server (such as Omni Sync Server), you can simply tick the box:
Reminders: [√] Publish Due reminders as a calendar
and then click the button:
( Subscribe in iCal )
Every time OmniFocus syncs, it will update that calendar to have all items due in the next two weeks. Then iCal/Calendar will also automatically sync and pick them up.
This thread links to what seems to be an
OmniFocus to Reminders export script.