Our friend from Norway has it right.
The system is in open loop until the controller recognizes a level of activity in the O2 sensor. Until it goes closed loop all mixture correction are mapped (programmed) and are made irrespective of what the real mixture is. Or in other words, in open loop the mixture will be the sum total of all mechanical and programmed enrichment.
This capability can be confusing. Many times a car runs better open loop than closed due to mixture irregularities cylinder to cylinder. A car with air leaks into individual cylinders or restricted injectors can find a rough idle or even single cylinder misfires when the average mixture is brought to lambda. The mixture that satisfies lambda has no room for error in the lean direction (plenty in the rich direction - remember this). Thus when the average is Lambda the lean cylinders can be in lean misfire. Some times the resulting misfires can add so much oxygen (unused due to the misfire) that the mixture gets richer - all confusing stuff when one is monitoring.
Bosch Master, ASE Master, L1
33 years MB technician