[CatOne]

Start dates and due dates in OF do entirely different things. Probably worth a read of the manual.

Note if you're using iOS that you have to enable an application to do notifications in Notification Center. Without that, you'll get no notifications. Have a look on your iPhone and iPad preferences under Preferences->Notification->Notification Center. If OmniFocus is at the bottom under "Not in Notification Center" you need to move it up.