I am not entirely sure, but that monitor uses an infrared beam to measure the fraction of circulating hemoglobin that has bound oxygen; which in this case has a bright red color. Basically, it measures the 'redness' of your blood. So when it sais 98% Oxygen saturation, it means that the level of oxygen in your blood is normal.
It can't give an indication of the concentration of oxygen in other body compartments, such as in organs, intestines etc. Because in pneumonia your lungs would be able to saturate your blood with oxygen less optimally, they would measure less in your blood.. so in this case it could be used to test oxygen absorption in in other locations than your finger.
In my opinion it would be interesting to see if Oxygen therapy can indeed reduce the Candida population, and if this would result in improvements of EC. However, I also wonder how much more we can improve the concentration of oxygen in our 'non-oxygen deprived' blood.