u dont have to shadow one to get in im sure. it is a bit of a negative to not have one though. but if you can get to an interview, they are probablly saying they are willing to overlook it for now and will then be asking you in your interview if you can tell them what you know about osteopathy.
something else to keep in mind, and i think its been said on here before, there ARE fewer DOs out there right now, so it IS harder to get a letter from one. but really, it is still quite easy, just call up your state AOA, and have them put you in touch with one who is willing to let you shadow him/her. it is real easy to do that, and shows interest beyond what the typical premed calling up a doc's office might otherwise be showing.