No, there isn't. However, you can put a project in another file where it will be invisible except when you have that file open. You could either just hide the file, or if you have serious trust issues, put it in an encrypted disk image stored on a USB drive that you leave with your divorce attorney for safe-keeping :-)
To make the separate file, do File->Export to export your database as an OmniFocus database, preferably naming it so it will be clear that it is not your regular database. Now double-click that file in the Finder, which will open it in an additional OmniFocus window. Delete everything (make sure the window title reflects that you are in the newly created file) and create your secret project(s). Close the window when you are done using it. Whenever you need to see that project you will have to double-click on that file in the Finder.