In July of 2012, Apple made changes in OS X Mountain Lion that interfered with the iCal Sync feature included in releases of OmniFocus for Mac prior to version 1.10.3. Because of this, the iCal sync option was removed. We apologize for the inconvenience this causes folks who were able to put the feature to use, but it had far outlived the intended original purpose, which was syncing to-dos with pre-iPhone PDAs. We didn’t want additional customers building their workflows around it.
However, there’s a separate feature with similar utility and which is still supported - it will put the due dates for your projects and actions on a new “OmniFocus Reminders” calendar.
If OmniFocus 2 for Mac is configured to sync with a server, you can enable this feature from the Notifications tab in the Preferences window. Just enable the “Publish due items as calendar alarms” setting, then press the Subscribe button once it activates; the app will walk you through the rest of the setup process.
Customers using version 1 of OmniFocus will want to visit OmniFocus > Preferences > Sync, then look for a similar “Publish Due reminders as a calendar” checkbox and button to subscribe Apple Calendar to the exported calendar file.
From now on, every time OmniFocus 1 or 2 syncs to your server account, it will also update the calendar file on the server with information on everything due in the next two weeks. This calendar is in a standard .ics format which is understood by many calendar applications, including Apple’s.