The O2 sensor is used as input to keep mixture in check. The mechanism of fuel metering changes but the act of the O2 sensor is the same for most cars.
The difference between open loop and closed loop just refers to whether the system is in control. On your car if you are worried about it then get it warm and disconnect the EHA or the O2 sensor - should give the same results.
Bosch Master, ASE Master, L1
33 years MB technician