The fine folks at Lynda.com have just made their OmniFocus Essential Training course available. They've got over three hours of material available, broken up into 5-10 minute chunks. You need a subscription to watch the entire course, but a number of the videos are available for free so you can decide if their approach will help you learn OmniFocus.
Haven't watched all the videos yet, but from browsing the titles, it's some pretty comprehensive coverage. GTD basics, due dates, syncing, perspectives...
For more info, head over to their
OmniFocus Essential Training page.