I, too, was diagnosed with severe OSA and have used a PAP machine ever since. Just a correction: CPAP won't help with atrial fibrillation if it was the original cause. Once your heart is disordered electrically, that's it...for life. What CPAP can do is to reduce the nocturnal strain on your heart that caused the AF in the first place. By keeping your airway splinted, you keep your optimal range of O2 saturation. The constant severe reductions in O2 in your blood during sleep apnea, with its constant threat of 'dying' , is what makes your heart disordered.....it can't get any rest at night. The CPAP keeps your blood oxygen levels up, and your heart can beat calmly, even down at the low rate of 34 BPM if that's what your O2 demand is and your heart is fit and strong.
So, your cardiologist is partly correct. CPAP won't make AF go away. AF is a progressive disorder (all the modern research makes this quite clear), and left unmanaged it can get worse progressively. The CPAP machine, apart from improving your quality of sleep by allowing you to STAY ASLEEP....not waking up gasping for air sixteen times each night... and therefore going through each successive stage of sleep properly..., will reduce the constant strain on your heart and possibly decelerate the progression you don't want. You have the disorder and it is permanent. It can be reduced in severity with CPAP, but also it needs strict management via drugs, lifestyle changes, catheter ablation (The Gold Standard of care for AF for most patients, especially early), or a pacemaker.
I said your cardiologist is partly correct. He's right on the face of it, as stated. But CPAP is necessary for longevity if you have moderate or severe sleep apnea. Not only will OSA cause sleep disruption, which is very unhealthy (weight gain, metabolic syndrome, kidney and lung problems, dementia, etc), but it causes your heart to continue, even more quickly, to deteriorate. CPAP is essential if you want to live longer, with or without atrial fibrillation.