As a promotion board member, how would you look at a letter to the board explaining why I only have one OER?

Sat a number of Navy active/reserve senior boards and recorded a Flag board. I won't presume the exact function of NG boards. However there are some absolutes. There are a limited number of slots, the size of the zone with above/below wiggle maybe, precepts, and procedures for conduct of the board. More important to your case is what the board can see. The problem is the not seeing part. 5 years ago? Who cares? One year of paper? No indication of trend, stick time, you name it. On the Navy side, there was a reporting senior's grading average which would be known to the board. A high grade from a Hard A is an indicator. Average from average says pretty much nothing. So you have a little that can be seen against competitors which can be seen more clearly. Your reporting senior of the one OER you have should have been savvy enough to know where you are WRT zoning and make a decision of what his/her one shot will do. I can think of several ways to make an OER difficult for a board to set aside, but many more ways where ignorance, not caring, not understanding the process, etc. can make a board conclude you need more stick time for a real look. It'd be a good idea to find out what items of your MC record are loaded into the NG side.

I immediately picked up on the school thing. Seems they had you go to the pedigree knife/fork school and are lined up for the next massage. Tells me you're at least worth that. Make sure anything pedigree gets done soonest. If you really want to max out the NG path, you'll need to get out to the badlands and get good paper on that. Key on senior grade promotion is standing out from the herd buy more than a few steps.

Now to the letter thing. There should be some guidance on what topics the letter can address. Stick to that and that only. Deviation tells board members that you either don't know how to follow instructions or you think they are too dumb to know why you are pointing out an obvious. There will be enough institutional knowledge on the board about why you and perhaps others are now appearing with limited paper. Don't assume the "why" is because they think you're a water walker. It's usually other mundane reasons; think business decisions. Good luck.

If you have a great deal of unrated time a letter explaining why, using existing orders and other official paperwork attached to your letter is fine. I do not believe that a letter from anyone else is authorized. I suppose you could attach it to your submission and see. I believe if it is obvious based on your submission you would get a fair review. I will further say that one year in comparison to what others will have detracts from your chance of selection based solely on rated time.

Seems like a good idea. Any record with unusual characteristics will usually be set aside for a second look during the review process. The board member will probably make all of the easy select/non-select decisions first, then go back and look at records in the gray area or with unusual stuff in them. If your one OER is top-block all around with a strong GO endorsement, you should have a good chance IMO.