Spo2 is hemoglobin saturation, pao2 is plasma saturation
SpO2 is the Saturation (peripheral) of Haemoglobin with Oxygen, expressed as a percentage ration compared to fully saturated Haemoglobin (Hb) i.e. when the Haemoglobin (the main oxygen carrying component of blood, found in red blood cells) is fully loaded with oxygen, then is it 100% saturated. It is SpO2, because this is normally measured by placing a probe on the finger which shines red light of two different types through the finger, and a calculation is carried out based on how much of each type of light is absorbed, which tells us how much the Hb is loaded up or saturated.
PaO2 is the Partial pressure