How have the rest of you fared in requesting letters of reference from doctors? Osteopathic schools strongly favor letters from D.O.s but I'm only acquainted with one, rather than a best buddy. These reference letters ask questions that assume a deep knowledge of the candidate's character, but how many of us know our professors and physicians that well?
Julie,
If you are applying to D.O. schools, then you certainly want a letter from a D.O. If you don't know your D.O. that well, see if you can spend a little time shadowing him. One afternoon a week for a few weeks, or maybe everyday for a week. I think you will find that you have a great opportunity to get to know him fairly well in that time, especially if you talk openly and ask questions.
As to the letter from an M.D. Go ahead and get that one, too. I know that when I applied to WVSOM this year, I ended up with three letters from D.O.s and two from M.D.s. Probably a little bit of overkill, but I worked with one D.O. and two M.D.s, and I had spent a week shadowing the other two D.O.s, and I wanted the school to know of my determination.
As to your professors, no, you may not know them exceptionally well, but take the time to talk with them. Take them a copy of your essay, your transcripts, your resume, and anything else that will give them an insight into who you are. And don't be afraid to listen. I know one of my letter writers this year insisted on talking about my husband's stroke and how it had affected my grades one semester. I had thought that was an issue I would address at interviews, if asked, but he felt that he wanted to include it in his letter, so I said go ahead. Well, it must have worked better that way, because I was accepted this year.
Anyway, good luck with getting your letters. I'm sure if you spend a little extra time with both the doctors and your professors, you'll do great!
When I applied to D.O schools that required a LOR from a DO, I provided one from a DO that knew me somewhat professionally and felt comfortable writing a letter. However, for any schools that didn’t require it specifically, I used a letter from an MD, and two Ph.D letters. These people knew me very well, the MD had supervised and was familiar with my professional work, and they could write letters that really described me. I am certainly not an Admissions expert, but I felt that the personal knowledge and credibility that having really worked with me was more important than the initials behind the name. However that is only an option if the school allows it 
First, I think that you will find that most DO schools are more interested in a physician’s LoR & not just a DO LoR – although, some of them can be pretty expectant of the DO letter.
Best way to get a great one…one that conveys a real sense of knowing & advocating you? Request a shadowing experience with the Doc, esp if it can last over multiple dates. Let him/her know up front that you are not only interested in the shadowing experience; but would also appreciate a “strong letter of endorsement” upon the conclusion of the shadowing. If he/she agrees, near the end of your shadowing experience, schedule an appointment with your preceptor for time to discuss your professional aspirations and offer him/her the opportunity to ask you more personal questions. In advance of this meeting, you should provide the physician with an up-to-date copy of your CV/resume and a statement of purpose/intent. Always strive to appear as professional as is feasible. In that light, for this meeting, I would dress business professional.