I don't know the data, but some take only step I or only step II. Many take both and some take neither. Honestly, I think very few DO's take the CS exam. It really has no chance of helping them. If they are bottom of their class, they are in trouble anyway and a passing score on an exam that 95-97% of MD students pass is not going to impress anyone. Some osteopathic schools (especially the ones opened in the past 10 years), in my experience, do a poor job of advising their students to be competitive in an allopathic match. On the contrary, many purposely try to get their students to only consider osteopathic and, in the past, have actively discouraged them from sitting for the USMLE at all, which puts them at a huge disadvantage. However, because they have no mentors, they don't realize it until it is too late or they sign up for the exam and are unprepared for it. These are typically the students in the third and fourth quartiles of their class, so it further submarines their efforts. I think the lack of mentors for the match process is the single biggest hindrance for osteopathic students because they are forced to figure things out on their own after being farmed out to small rural clinical rotations with private practice docs who are far removed from the process.
The outstanding top of the class DO students are self-starters who typically do not need someone to tell them how to stay on top of the process. They ace their exams and get excellent grades and great residency spots and turn out to be some of the best residents I have ever worked with. The bottom half of the classes could benefit greatly from a better mentor system, in my opinion.