Here’s an excerpt from the manual (available in OmniFocus for Mac at Help > OmniFocus Help) that should answer your question:
Restoring from a Backup
OmniFocus errs on the side of meticulous data preservation, automatically backing up your database once every two hours, in one of the following folders.
Omni store version:
Mac App Store version:
OmniFocus keeps up to 100 backups of your database at a time, which comes to about 2 weeks worth if you run OmniFocus continuously (more for most people, since a backup can’t happen if OmniFocus isn’t open).
To restore a backed-up version of your database, choose Show Backups from the File menu, then double-click to choose a backup file to view. It’ll open in its own separate OmniFocus window so you can browse it and determine if it’s the one you want; if so, choose Revert to This Backup in the notice bar and it’ll replace your current database.