Insofar as volunteering is concerned, it is best to start this as early as possible, especially if you plan on applying to schools (i.e. MSUCOM) where volunteerism and community service are very highly regarded. Most admissions committees don't require you to have an extreme number of activities, but rather a few that you have shown dedication to and have made a large part of your life. In the osteopathic profession, a lot of emphasis is placed on service and you will need to demonstrate this through your application(s) and interviews. If you feel that you are not strong enough in this department, I would seriously consider delaying your application process and try to get more service-oriented experiences. Remember, they don't necessarily have to be medically-related. Just choose something that you are interested in and feel good about! Best of luck!
Julia
P.S. If you check out the AACOMAS website, the deadlines should all be listed there.