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Go to OmniFocus > Preferences menu

Click on the Sync icon

You should see a setting labelled:

Reminders: Publish Due Reminders as a calendar. You can on the "Subscribe to iCal" once. Let it do its magic. It should create an OmniFocus calendar in iCal that will contain the due items.

However if you set the start date of a due item, it will only appear on iCal when today is the start date. Otherwise, it won't get published and you won't see it in iCal.