If that actually is just a draft, ask the doctor add to it in making the OSA connection on that same letter. He would be able to opine on the OSA,no matter what he treats you for. Focus on what you need done to make the connection and and just ask for it.
Berta is right, you might consider writing your own letter. If you can get the doctor to concur and sign your letter then it is just as probative as if he/she had written it. Better yet, ask the doctor to transfer the letter to their letterhead.
I have Pes Cavus and I though everyone's feet hurt all the time. It was when I got a job at the Post Office and after one long night on my feet I found I could not walk to my car that I knew something was really wrong. I knew that when I was in the Army I had a hard time with long marches and had knee problems. I think since my military service was 46 years ago it is too late for me. The VA considers Pes Cavus to be a congenital deformity. However, if your duty aggravated it and you filed within one year of ETS you probably have a good chance. One thing you want to avoid with Pes Cavus is weight gain. I had it when I was 6 foot and weighed 160 lbs. Now it is much worse since I am not that slim. The medical interventions to fix Pes Cavus are probably worse than Pes Cavus.
This is the draft he sent me to review as he has not written a VA letter before and wanted to ask if it would meet my needs.
I asked him for a nexus letter for sinus congestion/PND because the VA examiner said my OSA could due to sinus congestion.
I did not ask him for an OSA letter, because he does not treat me for that.
However, I am hopeful that if I connect sinus congesion, I can then connect it to OSA based on the VA Providers opinions.