[AmberV]

To truly start from ground zero, the best way would probably be to delete the OmniFocus folder in your Library/Application Support folder. This will wipe out everything except for your preferences.

If you want something a little less scorched earth, it could be more difficult. It would probably be easiest to delete all of the projects (make sure the filter settings are set to show all, not just active projects), and contexts. You might still have some orphan tasks without project or context assignment, though, and anything that may have been archived. As yet, there isn't any way to view the archives, so those will be there out of reach.

Disclaimer: I'm no expert.