how do I choose my officer branch at OCS?

From my understanding it’s mainly due to needs of the Army. But I have heard before that there is a merits list and those who are higher up on the OML gets their top preference choices. And 11A is one of the most competitive branches.

I just recently graduated from ocs and branched infantry. If your active duty (I can’t speak for reserve or national guard, there process is slightly different.) it’s based on a combination of your OML (Order Of Merit) Ranking, Preference List, and Branching Slots available as the slots available per branch each cycle varies. I will say infantry seemed to be the most sought after in my class. The higher you are on the OML the more likely you are to get your preferred branching. Every point counts. I have friends who went to ocs as national guard and reserve candidates who had there branch selected mostly before they got to OCS, but that’s about as far as my knowledge goes if your either reserve or national guard.

I'm not sure which component you're in or which you're referring to....but I'll answer for the National Guard side of things. You pick your top choices based on preference....but it all depends on what slots are available.

Active Duty:
I attended Federal OCS in 2015. Where you are on the OML has a huge factor in being able to choose. The higher you are on the OML, the more likely you’ll be able to pick your branch of choice. Each class will be given a certain amount of slots for each branch (based on needs of the Army). You are being evaluated everyday at OCS. From your Physical Fitness Test, written exams, to your evaluations (leadership; both garrison environment and field exercises) will determine where you are on the OML so do well throughout the course. IF you are prior service, you may be able to submit a packet to stay in the same field you were in as an enlisted. Ex: If you were an 11B or 11C, you can submit a packet to stay Infantry. Is it 100% guaranteed? No but from what I hear, those people have a 60% chance of staying in the same field.
Reserve Component:
Army Reserves: before attending federal OCS, you will have to get endorsed by a Reserve Unit. You will have to apply for a position and once a unit accepts you, you will receive a letter of acceptance which will have a “paragraph and line number.” It will annotate which branch you are going into. Unless you are part of the 442nd, you won’t be able to go Infantry.

National Guard: (for both Federal OCS and National Guard OCS) very similar to the Army Reserves with the letter of acceptance however, the needs of that particular states will ultimately dictate which branch you get. You may be told before attending OCS that you are going Infantry but when you return from OCS, that unit might have filled the slot with another person but needs an armor officer, they will send you to Armor BOLC. It all depends.

I went through many years ago and there wasn’t much rhyme or reason for branch assignments. A few prior service folks ended up with the officer AOC that corresponded with their prior enlisted MOS while others did not. I got AG. There was a board on the wall that you could post your branch and hope that someone would swap with you. I posted my branch, trying to trade to combat arms or engineer. Within a day I had about 30 inquiries seeking a trade. No combat arms and no engineer. I stayed AG and was quite happy with it.

Go Marine, every officer is an infantry officer... not reality, just a saying that holds some merit based on how the branches of service are different.