TVN - Don't concern yourself too much with the condition of oil burning and turning black UNTIL after you replace the O2 sensor.
Running excessively rich can wash down the cylinder walls, thin the oil and cause blow-by (the cause of the blackened oil).
Replace the sensor, verify no other codes, do a crankcase flush with something like Seafoam or other internal engine cleaner and give it a fresh oil and filter, then drive and monitor.
You may well find that life returns to near normal.