It would be nice indeed if you can find a private or VA doctor to give you a positive statement/opinion that your OSA is caused by or worsened by your PTSD and also state the same on a DBQ. I have to rely on medical treatment notes and test etc as none of my private or VA doctors will do this for me as they flat out said they will not take time of themselves and staff to do this even when I offered to pay for the DBQ. IMHO they are just lazy.
So here is what I did and this is my response on another forum "Although BVA decisions are non binding on VARO adjudicators I still use them among my other arguments and evidence I submit with my claims/appeals to the VARO/BVA as it makes me feel good. In order of importance I do list them and mention them toward end of my wordy claims/appeals processes that have been mostly successful. You of course must have qualified medical evidence/opinions to submit in your favor to the VARO. Here are just a few that the BVA granted Obstructive Sleep Apnea (OSA) as secondary to PTSD, etc. There are quite a few others that I have copied and saved and you can also find searching the BVA decisions on their website.BVA decision citation 1506771 and dated Feb 13, 2015. BVA decision Citation Nr: 1648003Decision Date: 12/27/16 Archive Date: 01/06/17. These BVA decisions along with CAVC court decisions almost always discuss how they arrived at their final conclusion/orders and this is most useful to me. A win by any means is still a win and thinking outside the box sometimes wins the day. Cheers and good luck to you.
There are also recognized reputable studies by outfits like Mayo, Harvard and others that conclude that Obstructive Sleep Apnea is caused and/or worsened by PTSD and somewhere I have saved this information to disk. These studies can also be used as medical evidence in support of your OSA claim. Note on another forum the VSOs and former VSOs are constantly telling vets that only being obese or overweight causes Obstructive (OSA) Sleep Apnea and that is pure BS. I am not an attorney, paralegal or VSO.