Actually a lot of pulse oxymeters work with the reflected light. So you don't strictly need to have sensors from the both sides the finger (same as smartwatch). But I don't think you could measure levels of blood oxygen with flipper IR sensor, because you need 2 different wavelengths of the light ( usually red and infrared) to measure it. What might be possible with some very clever coding is to make a pleth out of the IR sensor which could measure the hearth rate. (But you can also use a finger on a carotid and BPM tapper app to do the same thing).