Biomass is a popular feedstock in the pyrolysis process that allows the production of valuable fuels such as char, oil, and combustible gases. This study relates to 2D numerical calculations of oxidative fast pyrolysis of pine and pine bark in a drop tube reactor at 550 °C. Numerical studies were supported by experiments using thermogravimetric analysis of pine and pine bark. The study involved three different atmospheres of pyrolysis: pure nitrogen, 95% nitrogen/5% oxygen and 90% nitrogen/10% oxygen v/v. Numerical calculations were performed using Euler-Lagrange multiphase theory. According to received results, it was possible to linearly reduce the amount of solid residue from 49% to 34% for pine and from 57% to 36% for pine bark, as the oxygen content in the carrier gas increased from 0 to 10 % v/v. Furthermore, the addition of oxygen allowed to decrease the required energy for pyrolysis from 1.55 to 1.1 MJ/kg for pine and from 1.6 to 1.1 MJ/kg for pine bark at the maximum considered content of oxygen.