dont wait for anything and for the schools that accept the MD letter youll be complete right away and for the schools that only take the DO letter, you wont be complete there until they get it.
I had a similar situation: had an MD letter, but no DO letter. Some schools prefer a DO letter (but accept an MD letter), some schools don't care, and some rare ones require a DO letter.
I received an interview invite at a school that required a DO letter, even though I still didn't have one in. I interviewed, and simply had my decision withheld until I got the DO letter to them. It took me close to two months after I had interviewed to get the letter to them, and they sent the decision out promptly once they received it. (It was an acceptance.)
It will vary from school to school, and perhaps even from year to year (or between admissions committee members). The point is to say that it's not "game over" if you don't have that letter.
I'd say to send your application in, and stay on your DO mentor for that letter. Once you get it, send it to the schools you applied to as an application update. Even after you get the letter, stay in contact with your DO - again, I can't speak for every DO school, but the one that required the letter had a form with guidelines for what should be written, and many of them made it seem as if the letter should be tailored specifically to that school. I don't know how strict they are about that, but they wanted that form included with the letter.
Best of luck this application cycle!
