The aim of this work was to investigate whether the use of individual tree components (i.e., stem wood, bark, branches, and needles of spruces) as feedstocks during oxygen blow gasification is more efficient than using mixtures of these components. Experiments were performed at three oxygen levels in an 18-kW oxygen blown fixed bed gasifier with both single and mixed component feedstocks. The composition of the resulting syngas and the cold gas efficiency based on CO and H2 (CGEfuel) were used as response variables to evaluate the influence of different feedstocks on gasification performance. Based on the experimental results and data on the composition of ∼26000 trees drawn from a national Swedish spruce database, multivariate models were developed to simulate gasifier performance under different operating conditions and with different feedstock compositions. The experimental results revealed that the optimal CGEfuel with respect to the oxygen supply differed markedly between the different spruce tree components. Additionally, the models showed that co-gasification of mixed components yielded a lower CGEfuel than separate gasification of pure components. Optimizing the oxygen supply for the average tree composition reduced the GCEfuel by 1.3–6.2% when compared to optimal gasification of single component feedstocks. Therefore, if single-component feedstocks are available, it may be preferable to gasify them separately because doing so provides a higher gasification efficiency than co-gasification of mixed components. © 2022 The Authors