The operating principles of a fast paramagnetic oxygen sensor are described in detail. Some historical background of the paramagnetic effect in oxygen measurement is given as well. This sensor measures the difference between oxygen partial pressures in two gases as an alternating pressure signal being generated by the interaction of oxygen molecules with a switched magnetic field. Factors affecting the ideal performance of the sensor are analyzed. The main sources of interference are external pressure and vibration, and asymmetry in the transfer of the pressure signal to the microphone. Finally, the advantages of clinical breath-by-breath oximetry are briefly discussed.