Wait until you get the results from the Holter Monitor. Then you can decide if a moderate walking exercise program will be safe for you.
Unless your cardio doc did a stress EKG or echo cardiogram, the diagnosis of atrial fibrulation seems a little premature.
OSA does cause atrial fibrulation so it's not something to ignore. Make sure to get the full Holter report for your own records.
If you are still 150 lbs. over weight, then you should figure out a means to lose the weight. There is a really nice weight control program available called "Calorie King" that allows you to track all food and water intake as well as exercise. http://www.calorieking.com/
Weight loss for anyone is a matter of taking in fewer calories than you burn. This program will let you track everything so you can see on a day-to-day basis, exactly what's going on. BUT you have to record everything.
Do you have the software to get your own reports from your CPAP? If you post your equipment profile, others on this board may be able to offer better suggestions. Its helpful, when doing the profile, to select the TEXT option as many machines look just alike in their pictures.
BTW, I get 20-24 of those episodes every day and I don't have atrial fibrulation or any other cardiac problems. After very extensive testing both here at home and at the Mayo Clinic in Rochester, the docs are at a total loss as to the cause. For me, it's annoying, but not life-threatening. Mine started 9 years ago with just a few each day and has progressed to this huge number.