PaO2 represents the partial pressure of dissolved oxygen in the blood. In the UK PaO2 is measured in kilopascals (kPa). In normal subjects, the PaO2 is between 10-13kPa, although this can differ somewhat with age with it being naturally lower in older subjects.
However, in subjects where the PaO2 is less than 8kPa they are considered to be hypoxaemic (often termed hypoxic). In some subjects, such as patients with chronic lung disease, they may be perfectly able to function with a low PaO2 level. The cut off of 8kPa is deemed a risk factor and this person may be suitable for long term oxygen therapy.
The arterial oxygen saturation (SaO2) from the blood sample measures how well saturated the available haemoglobin is with oxygen. The SaO2 value should correspond closely with the SpO2 value measured by pulse oximetry. However, you must be aware that some slight differences may occur as the SaO2 is calculated, whereas the SpO2 is estimated. In practice the differences between these are negligible typically within around 2%. The normal range for both the SaO2 and SpO2 is between 95-100%.