I sat on an OCS selection board for the 101st when I was there. The board was active duty, and realize that everyone had been screened by the chain of command before anyone even got close to coming before the board. The board in this case was the last stop for anyone in the division that wished to attend OCS. I have no idea how it's done for USAR, but here's what we did:
There were three of us on the board. One Major and two Captains. We were all Combat Arms. The Major was Infantry. The two Captains were Aviation. As a side note, all three of use had been commissioned through ROTC. We had three selectees to interview. One was an Aviation Warrant Officer, one was an Infantry NCO from the Pathfinders, and the other was an Infantry NCO from one of the line units. Before the actual board, the three of us on the board reviewed the packets fairly quickly. There's not much in there that really we needed to know honestly, other than what they had written as to why they wanted to go to OCS. If they didn't have everything else squared away, it wouldn't have gotten as far as us anyway, so most of the packet was just reviewed for anything odd, or would standout.
There were no set questions. There were no trick questions. There were no bullshit questions. It was a simple, straight forward interview to find out just what we thought of the selectee, and what his potential was to be an officer. It was simple as that. We asked the usual, why's, what's etc, to get them talking and more relaxed. It's a high stress deal, and we all knew that, and it's something you take pretty seriously both as the selectee and as a member of the board, but you have to get the flow of communication going to see where the selectee is coming from.
That's about it. Expect questions like, "What kind of leadership experience do you have?" "What kind of problem solving skills do you posses?" "Why do you want to be an officer?" Some other general and specific questions, but none of the questions had a "right answer" per se. They were just questions to give the board an idea, or feel about your potential. Potential is what it was all about.
We recommended all three to attend OCS. I wouldn't say "rubber stamp". If you're going to get all the way to be one of the three guys going before the OCS board for the whole 101st Airborne Division (Air Assault), odds are you aren't a dirt bag. So it's easy to say if you made it that far, you made it. But frankly all three of us would have had no hesitation in not recommending someone we felt shouldn't go. It wasn't a "formality" to us on the board, or to the three soldiers who appeared before it. I know we took the duty seriously, and it was no rubber stamp. It's just easy to recommend people that are indeed what you'd like to see as an officer.
It's not the third degree. You'll be easily putting way more pressure on yourself than the board ever could. Just relax, be yourself and answer the questions without really thinking too hard or worrying too much.