Oximetry is a finger probe which uses an infrared light which passes through the nail bed and measures how much oxygen is in the blood. If a patient has obstructive sleep apnea then the saturation of oxygen usually drops in relation to the length of the apneaic event.

The oxygen-hemoglobin saturation of blood is then determined either by withdrawing a sample and passing it through a classical photoelectric oximeter (a photoelectric device that measures the amount of oxygen in blood and other fluids) or by electrodes attached to some translucent part of the body like finger, earlobe, or skin fold. It also includes non-invasive oxygen monitoring by pulse oximetry.