Yup, that shouldn't be a problem. The important part is to show that you have some exposure to clinical experiences prior to applying--I don't think most schools will care that the person who supervised you (I'm just assuming that this is how you know the D.O.) has a D.O. instead of an M.D. I had a letter from a D.O. and was accepted to allopathic schools this year, so it didn't hinder me, at least! 😉