You can access the backups on the Mac by choosing File > Show Backups in Finder.
You’ll find more information on backups in the OmniFocus manual (Help > OmniFocus Help). Here’s an exceprt:
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:
~/Library/Containers/com.omnigroup.OmniFocus2/Data/Library/Application Support/OmniFocus/Backups/
Mac App Store version:
~/Library/Containers/com.omnigroup.OmniFocus2.MacAppStore/Data/Library/Application Support/OmniFocus/Backups/