Mechanical biological treatment of waste is still one of the most popular methods for mixed municipal waste treatment. The result of mechanical processing of waste is sorting out: the undersized fraction from municipal solid waste (UFMSW) with granulation below 80 mm, rich in biodegradable organic waste (mainly including food waste, paper, wood, etc.). UFMSW is treated in biological processes in order to reduce the negative environmental effect of this waste. Unfortunately, the processing is not neutral to the environment. The correct course of the aerobic biostabilization process depends on the activity of microorganisms, the intensity of aeration, and the oxygen content in the processed waste. The aim of this paper was to analyze the effect of air-flow rate and biochar addition on the oxygen concentration in waste and in emitted gases during the intensive phase of UFMSW biostabilization. The study was performed under laboratory conditions. Six different variants of the process (without biochar addition and using 1.5; 3; 5; 10 and 20% of biochar addition) were applied. Subsequent replicates were conducted using an averaged air-flow rate of 0.1, 0.2, and 0.4 m3∙d-1∙kg dm.org-1. As a result of the conducted experiments, it was found that both the air-flow rate and the addition of biochar have a significant effect on the oxygen concentration in the treatment waste, as well as its content in the outlet air. Using the highest air-flow rate resulted in the oxygen content not decreasing below 14%, both in the free spaces between the waste and in the emitted gases, while the addition of biochar significantly reduced the oxygen concentration. In the case of lower air-flow rate values, the oxygen content decreased even below 5%. It was found that a high addition of biochar (10 and 20% by weight) at the lowest air-flow rate resulted in the occurrence of anaerobic zones in waste in the first days of the intensive process (between days 2 and 6 of the process), as well as absence of oxygen in the outlet air (between days 2 and 4 of the process). Despite this, no methane (biogas) emissions were found in any of the conducted experiments.