Thanks for straightening this out. I went from being convinced I was right to being sold on the fact that I was wrong, just to be informed that I was in fact right in the first place. I don't understand why I continue to use this website as a resource. So far my topic has had an entire discussion regarding AOC vs MOS and one individual bashing the HPSP program as a whole (which I explicitly requested remain out of this discussion) without receiving a concrete answer as to how one gets into their preferred AOC or the competitiveness of those slots. I suppose I'll have to go find a ex-HPSP doctor personally.
Alright, I'm getting the feeling that you're confused about some really basic stuff here, like how the medical education system works in and out of military medicine , and what physicians actually do in their respective specialties. I apologize if I'm wrong, but you're using a lot of borderline irrelevant terms like AOC and MOS and you aren't really using the terms like 'specialty' and 'billet' that would indicate you know what you're asking. So, to give a primer
How to become a doctor:
1) The first step in becoming a physician is to get an undergraduate degree and finish your premedical requirements, and then go to 4 years of medical school and become a doctor. However a 'doctor' who graduated from medical school can't actually do anything yet, he needs to go through another 3-10 years of training known as residency.
2) Your residency is where you train for your specialty, which means whatever kind of doctor you want to be. Residencies, like medical school, are something you need to apply for. Some are competitive, like Neurosurgery, and you need a really good application to get in. Others are open to almost any graduate, like family practice. You apply to residencies in your last year of medical school, through a process called the match. If what you're asking is how competitive the match is, you need to tell us what specialty you're trying to match into. Even then, the answer might change by the time you get here.
3) The military has its own, internal residency system, and when you take the scholarship you need to apply for military residencies through the military's own match. Just like the civilian match your odds of getting what you want depend on your grades, your standardized test scores, and how well you interview. The only difference is that the odds of getting some specialties are much worse (for example Pediatrics is more competitive) and the odds of getting some specialties are slightly better.
What you can do in a warzone:
The reason so few people here know their AOC/MOS/whatever is because these terms aren't that important to us. In military medicine what you can do in combat is the same as what you're credentialed to do out of combat. Some examples:
1) If you want to work in a warzone, and put bleeding/burned/blasted service members back together again, you need to be a Surgeon. To be a Surgeon in a warzone the only requirements are that you did a surgery residency, and you were willing to go/couldn't weasel out of going to a warzone.
2) If you want to go to a warzone, and put those same injured soldiers to sleep so that the surgeons can fix them up, you need to be an anesthesiologist, which means you need to have finished an anesthesiology residency
3) If you want to just provide primary care for service members in warzones ('hey doc, what's this rash on my ***?) you can do a residency in anything. A primary care job for a battalion is called a 'battalion surgeon'. There is, as previously noted, no surgery involved. There's just administrative work, corpsman training, and sick call.
BTW if you want to crawl out into gunfire and drag wounded soldiers to the safety of the line like the 'doc' in band of brothers... that's not a doctors job. That's a medic/corpsman, enlisted medical personnel who work a lot like EMS in the civilian world. Doctors do other things. Watch MASH for a good overview.
How you get your billet (i.e. job)
So how do you get hooked up with the marines to go to a warzone? The key word here is billet. Every 2-3 years you apply for a new 'billet' in the military, which means a new job. You are given a list of jobs you qualify for by a person called the detailer, and then you bargain with that person for the job you want. For example if you're a surgeon they might need one person to provide care at a hospital in Baghdad for a year, another person to cover the surgery service at a family practice residency in camp Lejune, and a third person to handle surgeries at a major medical center like the big hospital DC. Billets are not 'competitive' per se, there is no objective merit based application process to decide who gets what. Anything that's far or dangerous, though, is generally something the detailer is happy to give away to someone who asks.
So if you want to do surgery in a warzone you need to do well enough in school to get a surgery residency, then complete the residency, then ask to go to a warzone.