What fixes low oxygen level at night?

Depends on cause...: Most often, this is related to obstructive sleep apnea. This is when your airway becomes "floppy" while sleeping. The airway closes and no air can pass into the lungs. Therefore, the oxygen level drops. Using oxygen at night won't fix the problem if this is related to sleep apnea. You will need a special machine that increases the pressure in your airway while sleeping. See a sleep doctor.

Sleep apnea: A CPAP ( continuous positive air pressure) device is the standard of care for severe low oxygen levels. For mild to moderate decreases, see a dentist who is trained to custom make a device that holds the lower jaw forward, which opens the airway to allow more oxygen. Losing weight, evaluation for allergies, an ENT evaluation to ensure no blocked airways are alternatives.