I had to have a sleep study done shortly after my diagnosis of PAH. Although the pulmonologist was fairly certain my PAH was caused by an autoimmune disease I have, she said she had to rule-out other causes. She also said a significant number of people with any form of PH have sleep apnea too. Even if PH is not caused by sleep apnea; sleep apnea can increase pulmonary pressures and/or make your heart work harder and prematurely increase mortality incidence.

Quite often sleep apnea affects PH and thats the reasoning he probably has for wanting a sleep study. Have you worn an oximeter at night to see your levels? If so that would be reasoning for nocturnal O2. I wear O2 24/7 and at night if you move the ring around your tubing up it makes it easier to stay on.
These are questions to ask your cardiologist.

Sleep apnea can co occur with PH & can make breathing even more difficult

I actually wear the cannula on top of my head at night. I just adjust the tubing on top of my head snd it works perfect for me.

I was diagnosed with sleep apnea just months before my PAH diagnosis. With PAH I am on oxygen 24/7. The oxygen is connected to my bipap machine. I don't even realize I have any of it on at night. I have become very adapted to it. I know that is not the norm as most people have a lot of problems with cpap and I am very grateful. I am very blessed.