Depends on the schools you're applying to. Some require a DO letter and really like to see that you've spent time with a DO. Others aren't so particular. You might make a call to some of the admissions offices and see what they say.
I shadowed both DOs and MDs, and had letters from both. My school does require a DO letter, but they'll take additional letters from MDs. I don't know what their stance is on the shadowing, but many people ask the Drs they shadow to write them letters, so that is why I've gone off on that particular tangent. 🙂