Part of "doing GTD" is doing the GTD daily review. :eek:
As part of this process, you move stuff that is fixed in time over to the calendar, such as appointments and whatnot.
So if you're using OF the right way* you don't need OF to beep at you and tell you about appointments, because the appointment's already in the calendar and you've made sure you've got the time set aside to travel there (and back).
This is why you do a daily review - load up your reminder system for the stuff you have to get done in the next day (and a bit).
If you haven't got Dave's book yet, make sure you
go grab a copy and start reading.
Hope this helps!
* The right way being my way, of course ;)