Make a backup now, if you haven't already. Then contact our support ninjas by email (
omnifocus@omnigroup.com) or telephone (1-800-315-6664) so they can help you recover your lost project. What we need is a backup made after you synced the changes from your phone, but before sync history gets coalesced.
If the backup is made before your sync history is coalesced, the "delete project" change from your Mac will show up as a separate sync history entry from the "add new items to project" changes made on your iPhone. If that's the case, we'll be able to nullify the "delete project" change in your sync history and you'll get all your new items back.
(If you had added another item to that project on your phone
after deleting it on your Mac, the "last change wins" policy would have caused that project to resurrect itself. But since your last change was to delete the project, that change won.)