It's always best to contact specific schools with questions like this. In addition I don't know how the college you come from operates. I know that if your college does a committee letter, like Penn State, then that would most likely be the easiest. Essentially you get three or so letters from professors, bosses, etc and send them to your school's advisors who compose one good letter of recommendation which covers all the aspects from the other letters. As for the letter from a DO you will have to find out from the specific school. However, I can tell you that most schools only say that it is recommended not required. I never worked with a DO and did not have any difficulty without the letter. My advice if your don't have a DO letter would be to use your essay to explain why you want to be an osteopathic physician. Then make sure you know a good bit of the philosophy for your interview.
-Chris
