Put a machined guard around the pulse oximeter to keep the surface from it at about 0.2-0.3 (if it is and SMD part soldering height tolerance is about 0.2mm) and then simply screw down the resulting assembly.
Otherwise use weak springs to push the p.o. against the case, need to know the allowed stress on the body for that.
Read More: Medical Electrics Manufacturing
#PCB Assembly
https://www.youtu.be/kX4gCs9szeA?si=_o1w0lNIu4HwRX7N