OmniFocus is my task manager of choice, as explained in my review. It's a sophisticated application and, if used correctly, it may become essential to your work. Personally, I depend on it. It is worth investing time, effort and money into mastering something you could depend on too.
So, I've put together a list of third and first party resources which will help you to get the most out of the application.
Third Party Resources
Learn OmniFocus by Tim Stringer is designed to help people get into the habit of using OmniFocus quickly and easily. Membership is very reasonably priced at $59 / year. Tim runs a regular series of live webinars for members, hosts recordings of past sessions, plus a large library of videos and articles. This is a superb resource for anyone who wants to get up to speed with OmniFocus quickly and easily.
OmniFocus Field Guide by David Sparks is a brilliant resource, especially for people who prefer to learn by video. It's well-structured, well-produced and reasonably priced at $9.99. I think it's one of the fastest ways to get competent as using OmniFocus. Still, experienced users will find plenty of great information, including tips on enhancing OmniFocus with TextExpander and scripting.
Creating Flow with OmniFocus by Kourosh Dini is a comprehensive, highly detailed and very thoughtful masterclass in turning OmniFocus into a trusted system. This is essential reading for anyone who wants to use OmniFocus as a brain in their pocket.
SimplicityBliss by Sven Fechner has a library of rigorously thoughtful resources, great for going further with OmniFocus. In particular, take a look at his ideas for custom perspectives, and his fresh take on contexts. The latter uses energy levels and time to describe a context in terms of emotional resources. It's a great idea. It doesn't entirely agree with my approach but it'll be useful for anyone who finds themselves spending most of their day working at a desk.
Getting Things Done by David Allen is one of the most profound and influential books on personal productivity ever written. It inspired me a great deal, and provides a framework on which I've developed my own ideas, including many which you'll find in my book. GTD formed the basis of OmniFocus's methodology. While it's possible to use other productivity systems in OmniFocus, I think it's best to use a GTD-derived methodology. Essential reading.
The Productivity Habits by Ben Elijah. I know, a shameless plug. But many of the ideas in the book can be directly applied to OmniFocus, such as the Importance Tree, the Context Triangle; and handling perspectives, reviews, and start/due dates.
Omni Group's Resources
Omni Group have a really useful introduction to OmniFocus on their website.
I also find their Inside OmniFocus pages equally useful, not least because they present a selection of different workflows by productivity experts.