Where you hook the volt meter depends on what you want to see. If you wish to see system performance one must monitor the voltage with the O2 sensor hooked to the system.
Measuring the open circuit value is only of use in determining whether the sensor CAN work and in determining whether mixture is VERY rich or VERY lean.
If the car runs in closed loop it is probably very close to right. Closed loop can only be established with the sensor in the system by definition of "closed loop"
Bosch Master, ASE Master, L1
33 years MB technician