Main problem is variability of oxygen availability with stirring or movement of the electrode. If the electrode is made very small, then the surface layer does not move. If the electrode is coated with a permeant layer, this defines the diffusion distance. Either a membrane over an electrode, or a recessed electrode. Thicker undisturbed layers lead to slower response times.
Oxygen reading instruments simply monitor the current output of the sensor.
Oxygen consumed is determined by oxygen that diffuses through the membrane. Fuel cell oxygen sensors are used up over time. Even when installed in an instrument which is never turned on, oxygen sensors which are exposed to atmosphere which contains oxygen are generating current, and being used up. Must be recovered or disconnected. Paramagnetic sensor Oxygen is paramagnetic, and measuring the force of attraction of a gas filled vessel by a magnetic field can be used to measure oxygen content.