Reminders in OmniFocus - Tip!
 

I came up with a pretty good way for creating a timed reminder in OF.

Select a task that you want to be reminded of, say two days from now at 12.30. Put the start date and due date to the same day and time. Now all you have to do is wait. When the day arrives, at exactly 12.30 the project/action will reveal itself and it will immediately be turned red.

How well this works depends on how often you watch OF. I pretty much live in there during office hours so this works for me.