Brother,
If a psychologist know's you're rated for PTSD, normally they, if their worth their pay, know that Sleep Apnea (OSA) can be a secondary issue to PTSD. (Many won't connect that dot, unless you mention it and/or they've had a sleep study done!)
However, just for your info and to help you, you will need the complete study, from a sleep lab and their opinion-to make sure the OSA isn't genetic.
Just tell them about your sleeping issues during the military and that you're a combat vet, all the meds you take, tell them you're diagnosed with PTSD and that you've had sleeping problems since__ you insert that date.
Tell them you've been informed that Sleep apnea can be associated with PTSD and ask them to give their "expert medical opinion" on your condition.
Once they confirm and give that to you, show it to your shrink and see if he/she too will give you a confirmation that the Sleep Apnea is secondary to PTSD.
Submit that documentation and it should grant you service connection for SLeep Apnea, secondary to PTSD. Without a problem or stress.
Hope this helps.
Whoop