Apple has made changes in OS X Mountain Lion that interfere with the iCal Sync feature in OmniFocus. These changes include deprecating the Sync Services API and converting local calendars into iCloud calendars. Depending on your exact calendar setup, it may misbehave in a variety of different ways.
Because of this, the iCal sync option has been removed in the latest update to OmniFocus for Mac. We apologize for the inconvenience this causes folks who were able to put the feature to other uses, but it has far outlived the intended purpose, which was syncing to-dos with pre-iPhone PDAs. We don’t want additional customers building their workflows around this.
However, there’s a separate feature to put the due dates for your projects and actions on your calendar. If OmniFocus is configured to sync with a server, on the OmniFocus > Preferences > Sync tab, there’s a checkbox to “Publish Due reminders as a calendar” and a handy button to subscribe to that calendar from Apple Calendar. With this enabled, every time OmniFocus syncs to the server, it will also update a calendar on the server with everything due in the next two weeks. This calendar is in a standard .ics format which is understood by many calendar applications, including Apple Calendar.