Mass transport properties of the oxidation and gasification agents O2, CO2, and H2O are highly relevant for the modeling of the conversion process of biomasses. Therefore, this study presents experimental investigations on the adsorption kinetics of O2 on a biomass char using a modified gravimetric sorption device. Based on this comprehensive set of adsorption kinetic data, a first parameterization of the pore-structure dependent kinetic adsorption (PSK) model for O2 adsorption is presented. This model intends to account for mass transport during biomass conversion in a more meaningful way as it is considered in conventional conversion models. With this parameterization, the model is capable of describing accurately the adsorption kinetics of O2 as a function of time, temperature, and pressure.