The method anamorph suggested would work, but there is also an easier way to do this. It is possible to create a separate non-syncing database. To set that up, you would need to create an OmniFocus Document, which behaves just like a regular OmniFocus database, only that it cannot sync and can therefore only be used on one device.
The process for initially creating an OmniFocus Document is a little tricky, but once created functions just like a regular database (aside from not being able to sync). Here’re the steps to set that up:
• Select Menu Bar ▸ File ▸ Export… and change the File Format to OmniFocus Document, choose a save location (such as your Desktop) and click Save.
• Now, in Finder, go find that file and open it. It should display with a blue banner identifying it as an OmniFocus Document (this is how you know it’s not your main/syncing database). At this point it is essentially a copy of your entire database. Probably the easiest way to achieve your goal would be to delete all of its contents and then copy/paste or drag/drop from your main database into this window to create a database with only the items you wish to keep separate.
• To delete the contents, select Menu Bar ▸ Perspectives ▸ Changed, then select one project in the sidebar of the Project perspective tab and use Select All (⌘A) to select everything, then press the delete key ⌫ on your keyboard. Repeat that process on the sidebar of each perspective (Inbox, Contexts, etc.), ensuring that the View is set to show All, until nothing remains.
• If you’d like to keep multiple separate OmniFocus Documents, you may want to duplicate this blank file now for future use.
• Once you have an empty OmniFocus Document, you should be able to copy/paste actions and projects from one window to the other. Copy/pasting a project will transfer all corresponding actions, contexts, folders, etc. along with the project, including completed actions (so you may want to clean that up as you go).
• Once the OmniFocus Document looks good to you, you can use it for anything that you don’t want/need to sync.
An OmniFocus Document can also be emailed to others. To do that, close the OmniFocus Document window, then compress the file in Finder before attaching it to an email (to do so, control-click on the document in Finder and select the ‘Compress {filename}’ option from the contextual menu). Your recipient should be able to then add the contents of the OmniFocus Document to their OmniFocus by copy/pasting into their database window.
Hope this helps!
~Anne