Obstructive Sleep Apnea is generally caused due to relaxation of muscles of the tongue and soft palate.
The common symptoms are like snoring, excess sleep, headache, high blood pressure, facing mood swings as depression.
Prevention can be depended on how your apnea is, mild or severe. If you have mild obstructive sleep apnea then you can treat with lifestyle modification/changes such as losing weight in case of overweight, regular exercising, quit smoking, do not sleep on your back. If apnea is moderate to severe then surgery is the option.