[Brian]

OmniFocus supports "Undo", so that's the easiest way to recover from something like this. If that's not possible, don't give up; OmniFocus on the Mac will periodically back up your database.

If one of those backups occurred before you deleted the project, you can retrieve it. (You can specify how frequently this occurs in the "General" pane of OmniFocus' preferences.)

On your Mac, open up the following folder:
<your home folder>/Documents/OmniFocus Backups/

In that folder, you'll see a bunch of time-stamped backups of your database. Double-click the most recent backup; it'll open a new OmniFocus window alongside your "real" database.

If this backup was created before you deleted the project, just drag the project from one window to the other and you're set. If it's not in this one, close the backup window and try the next most recent one...