Unless they have changed the format (and I haven't heard of anything like that yet), you're usually interviewed by anywhere from 1-3 members of the faculty and maybe administration (someone will have to check me on that). Depending on the interviewers, one may get some difficult ethical and/or provocative questions or the questions may all be rather routine and laid-back. I suggest being able to talk about anything mentioned in your secondary application (since they will have it sitting in front of them) as well as prepping with the usual sort of interview questions that get asked. These sort of questions may range from listing your strengths and weaknesses to your knowledge of osteopathy to why do you think you would be a good match for this school to questions regarding your feelings between going to a DO or MD school (Just to list a few examples) Personally, I think the main key is to be genuine and not to be too stressed out. Sometimes, it is not a matter of having the correct answer as much as having a thought process that was well-thought-out.
Good luck and try to relax and enjoy your interview day!