How to move my OmniFocus data from old Mac to new Mac?
Quoth OmniGroup:


  The database file now is in a sandboxed container. The easiest way to get to the file is by opening the backup folder through 'File' → 'Show Backups…'. The data file is in the same folder as the Backups folder.


From: https://discourse.omnigroup.com/t/where-does-omnifocus-2-keep-the-database-file/4211/2