Biochar produced at an oxidative atmosphere shows distinct chemical properties compared to those of biochar produced at an inert atmosphere. However, there has been little investigation on the relationship between the oxidative atmosphere and the structure development of biochar, which can be useful information for the utilization of derived products. In this study, the effect of the oxygen atmosphere on the structure development of loblolly pine-derived biochar during thermal treatment was investigated. Quantitative analysis using solid-state direct polarization/magic angle spinning 13 C nuclear magnetic resonance spectroscopy presented the existence of large fractions of aromatic and non-protonated carbons in the biochars treated at an oxidative atmosphere, implying highly condensed aromatic structures with large cluster sizes. Simultaneous thermogravimetic analyzer-differential scanning calorimetry was employed to demonstrate the difference in heat flow during the thermal treatment at two different atmospheres. Relatively large exothermic heat flow was observed when woody biomass was treated at 350 °C under an oxidative atmosphere which might be responsible for the difference in structural alteration. The series of observations suggested that compared to inert atmospheric conditions, admitting a limited amount of oxygen during thermal treatment of woody biomass could promote the evolution of condensed aromatic carbon structures.