Dear 100 Hour Board,
If you were me (count your blessings that you're not), would you go to DO school, and matriculate this Fall, or would you get a Masters (to boost the app), retake the MCAT, and apply to MD school in two years?
For me, personally, that would depend on a lot of factors. The first to consider is whether having an MD instead of a DO will really make a difference to your personal career goals. Do you have your heart set on a competitive specialty, pursuing a rigorous and competitive residency spot, having a career in academics or research, or being involved with big-time leadership in a hospital or medical school? It's not like those things are completely barred to DOs, but if any of those things are important to you, having an MD would definitely help open some doors for you. However, if you're interested in less-competitive specialties, and you mostly want to dedicate your career to helping patients and providing for your family, and not necessarily climbing the career leader, then having a DO instead of an MD becomes a much smaller deal. Choosing your degree program in light of your career goals is definitely the first thing you need to do.
My other advice applies if you decide to hold out for a spot at an MD school. First, you need to find out what held you back from getting into MD schools this time. Was it really just your MCAT score, or were there other areas of your application or interviews that needed improvement? If you haven't yet, you should definitely talk to someone in the Pre-Professional Advisement Center about ways to strengthen your application. I also recommend talking to the admissions offices at schools you were rejected from and asking about ways to strengthen your application for next year—it will be enormously helpful to you if you can find someone to be brutally honest with you.
If you talk to admissions offices and it turns out that your MCAT score was a factor in rejections, you also need to think about how likely it is that your score will improve next time. Did you make an obvious mistake this time around, like not taking preparation seriously enough? Or will your score in a year or two be similar to this time around? Consider that if you're even further removed from beginning physics and chemistry, things may be harder the second time around. Before you hold out for an MD school, you need to be sure that you really can improve your application enough to be competitive; otherwise you'd just be wasting time.
Finally, if you do end up deciding to wait and re-apply, then you should definitely think about what you'd like to do during your break and how much it will really help your application and/or future career. For instance, if your undergraduate grades were so-so and that hurt your application, then working hard to get excellent grades in a master's program could be make a real difference in showing you can handle difficult coursework. Same thing goes if you need some stronger letters of recommendation—master's programs can definitely help you get to know some faculty members and get some great letters. Additionally, a master's degree would be a good choice if it would really prepare you for aspects of your future career (for instance, if you envision yourself working with community health later in your career, then spending time getting an MPH now could be very helpful).
However, master's degrees are a dime-a-dozen among med students, so getting one just to get one doesn't look impressive to anyone. Remember that if there's no specific area of your application that a master's degree would improve, then getting one in the vague hopes of looking better won't help you. Let the weak areas of your application determine what you spend your time doing. If it was really just the MCAT holding you back, then it would be better to do some serious MCAT study and ace it this spring, which would only put you a year behind schedule—meanwhile, you could find a job somewhere for a year and save up money for interviews and next year's tuition. If you're shy on clinical experiences, then getting a job for a year in some medical field and doing some solid shadowing might be better than a random master's degree. If, as I said above, your undergraduate grades are not great, or you didn't have good letters of recommendation, then a master's or post-bac program actually could be really helpful, but remember that having a master's degree isn't really helpful to your application or particularly notable in and of itself.
tl;dr Talk to the Pre-Professional Advisement Center and some of the schools that rejected you and figure out what will make you a better candidate for the next application cycle. Do whatever will help strengthen your application and advance your career goals, whether or not that includes getting a master's. (Obviously, this is IF you decide that holding out for the MD will be worth it to your career—otherwise, congrats on getting in and good luck this fall!)