I'd say it's dependent on school culture. I went through the CIB and nixed any schools that required or strongly advised a DO letter of rec. I got plenty of interviews after using my MD physician letter and it never came up.
One school asked me how I learned about osteopathic medicine, or what kind of exposure I had to it, or something like that. I told them it took me years to realize my family practice physician was a DO and that, at least in how he treated me, I couldn't see any difference between him and the MD's I had seen in the past.
Edit: My follow-up point to that question was that, difference or not, I thought my osteopathic doctor was a very competent doctor who was always kind and showed a lot of respect for his patients in how he treated them, and that that was the kind of physician I wanted to become. Not directly pertinent to the conversation I know, I just didn't want to give the impression that ending the discussion at "I don't see any difference" was a successful strategy.