Keep in mind that almost all schools require a letter from a doctor, and many of them require a letter from a DO. These letters often come from shadowing or volunteer experiences, but not always. I have gotten interview invites from all the DO schools that I applied to without a DO letter, but with 8 hours of DO shadowing. They might ask you about your shadowing or volunteer experience in your interview, so just be prepared to explain why you haven't be able to do it. Best Wishes!