As an Officer Candidate you're at the mercy of the National Guard. It's not like the enlisted side where if you don't like the MOS slot that's open to you at the MEPS station, you don't sign the papers. As an officer you take what you get and do your duty. The state fills 2LT slots with OCS and ROTC graduates, so each commissioning source is given a proportional amount of paragraph and line numbers (equivalent to job codes in the civilan world) to fill.
The exception to that rule is that you can be a go-getter and find a 2LT slot in a unit and branch you want and get that commander to put it in writing and send it to your state HQ. This can be done at any time up to the time you leave for BOLC. Just don't expect it to be an easy route and there are still no guarantees it will work. We had a female in our class who secured a paragraph and line number only to be re-assigned by the state. On the other side of the coin, we had someone who changed branches after being assigned (which you can do before BOLC begins).
Two months after the branch brief, we were given our branch preference sheet. There are 16 branches available to junior officers. The branch preference sheet asks basic questions like where you live (with the option of asking if you're willing to travel over 250 miles to your unit), what your degree is in, which unit(s) you've been a part of (they won't let you go back to any of the units you were serving in as an enlisted soldier, even if you were just there for one drill between BCT and OCS), and as the title suggests, what your preference is for branching.
Of the 16 branches, Aviation and Medical Service Corps require that you've already been working with a recruiter from that branch, so start that as soon as you get into OCS if those are routes you wish to pursue. From there you number your top 13 choices from 1-13. Males must have three combat arms branches in their top five slots.
We turned those in April, and in May we were given our branch and unit assignment. What happened in between, no one really knows.
After OCS I'll be moving on to an Engineering unit- I'm excited about that. Initially, I didn't want to do what I did on the civilian side, but over time I realized its what I was good at and professionally it won't hurt to have parallel career paths. It took about two minutes for us to embrace our branches and the banter carried on like we've been doing those jobs forever.
We've spent a vast majority of our time in the field the last two drills. That will make for much more interesting reading and I intend on summarizing all that into one post. As a class, we know we're close to being done with this, but as a whole we haven't started acting like "short timers." We're still working hard to the end, it doesn't get any easier and there are sill a few key hurdles in between us and Phase III. And though no one will admit it, we're getting nostalgic. We just have two drills left with this close knit group before we're all sent on our separate ways. We've developed close friendships and they're ones I'm exceedingly grateful to have.
Feel free to shoot me any questions that weren't clearly answered in the comment section and I'll to my best to answer them.
Here's my response to that:
A) Go active duty, the weekend warriors don't get the budget to do cool stuff. Best I can tell we're the B-team.