First and foremost, take a deep breath. You'll be good. Don't psyc yourself out, focus on your MCAT!
In terms of the DO letter, you could use an MD one as a replacement, have you shadowed an MD?
What does your app look like? GPA, MCAT (what are you aiming for), EC's. Will give us a better idea of whether or not you'll be competitive applying a bit later in the cycle.
I'm writing my MCAT in 2 weeks, and my letters are gonna come in mid-July.
Submit your AACOMAS asap with your transcripts, extracurriculars and personal statement included. As long as you have specified the person that will be writing the letter, they can submit it after your primary. Your ideal goal is to have your letter done around the time you submit your secondary.
I'm fairly new here but this is what I've been getting from people. Someone correct me if I'm wrong. I dont want to be misleading.