Personally, I would urge you to have a letter from a DO that you've shadowed. It shows that you've had contact and exposure to osteopathic medicine. From what I've heard, the only reason you should have an MD letter in place of a DO letter is if the MD knows you very well and can speak of your qualifications moreso than a DO, or if there are absolutely no DOs in your area. Either way, you should try your absolute best to shadow a DO to gain first hand exposure to the profession and have him/her write you a letter of rec. If I were on an adcom, I would be a bit suspicious if you didn't have a letter of rec from a DO. My two cents.