I use a CPAP (Respironcs System one Series 60 ). I have been using it for the last few years and its changed my life. I have had OSD for a few years but even when I was thin I had a snoring issue.
I use a Nasal mask with full head gear, not the pillows, though pillows are more comfy ( I have a set ). I am a nose breather, always have been. So for me the mouth stays closed. However the mouth may open when you have difficulty breathing (if the pressure is set wrong etc).
As for dry throat, its due to the humidifier not working correctly. You got to put distilled water, and change the water every day, and then rinse it once a week. Though I've gotten lazy and i go a few weeks between wash/rinse, but the water is changed out every night or so. I find the humidifier works great for me at mid level, the only time I need the high humidifier if I am in vegas.
Remember you can be more prone to sinus infections by not using the humidifier, i learned that the hard way after 4 sinus infections one after another. THe CPAP is a great device. I can sleep 6 hours and be calm, relaxed and confident in my day to day activities, since i am not tired and cant think.
1. Try a different mask - request they give you a full mouth mask or a nasal mask.
2. Use it everyday, force the use, there is an initial discomfort, but once you get used to it, about 30 days, you cant live without it.
3. Learn to clean it regularly and get regularly new supplies. Use both ultra-fine filter and the pollen dust filter.
4. Get a battery pack for travel (I use a CPAP Super C-100 battery from cpap.com)
5. Use the Ramp button if you got one, to lower pressures and have it gradually increase it as you fall asleep. If you wake up, use the ramp feature again..