What is the difference between O2Sat and SaO2? Both appear to indicate oxygen saturation levels.

O2Sat and SaO2 mean the same thing although the more popular denotation is SaO2.

SaO2 refers to the amount of oxygen bound to hemoglobin in arterial blood. SaO2 can be measured either by Arterial Blood Gas (ABG) analysis or by pulse oximetry.

SpO2 is an indirect measurement of the oxygen content of blood measured using pulse oximetry, whereas, SaO2 is a direct measurement of the oxygen content of the blood (ABG sampling).