Loss of oxygen while slepping.. what condition causez this?

Sleep apnea: I presume you are referring to the intermittent episodes during sleep in which breathing stops for many seconds followed by resuming. Sleep apnea is diagnosed by sleep studies and imay be treated with appliances, such as a CPAP mask, which is comtinuous positive airway pressure mask helping to keep airway passages open and allowing normal breathing. Other treatments are also available.