I don't feel like it is too late. I am a current second year DO students and I had classmates apply later than this. Just make sure you can answer "why do you want to be a DO". If it is solely a "back up" and you really don't want to learn osteopathic medicine, I'd advice not applying. However, if you are actually interested in OMM or at least willing to be interested in it, apply. I am 100% glad I chose a DO school (I applied MD and DO). It is nearly a 100% guarantee that your secondary application and interview will ask you about OMM (at least something like why did you chose DO). And please (for the love of God) don't tell your interview or application or students you interact with at a DO school that this is just a backup. If the right (or wrong) person hears that, it could ruin your chances of getting into a DO school. I wish you luck in your endeavors.