This study investigates the synergistic behavior between oxidative atmosphere and potassium catalytic effects on woody (Amapaí) and non-woody (Miscanthus) biomass. Inert and oxygen-lean torrefaction was conducted at 275 °C for demineralized and K-loaded samples in fixed-bed and thermo-gravimetric equipment. Torrefaction improvement was assessed by torrefaction severity index (TSI), Catalytic Enhancement Area (CEA), Catalytic index (CI), and conversion rate (CR). Gas chromatography-mass spectrometry analyzed the impact on condensable compounds. The K% anticipated the biomass oxidation and the oxidative medium improved the catalytic effect. CEA and CI provided a reliable performance index (R² > 0.90) to assess the synergistic effect. Condensable compound yields variation with increasing K% was similar for both atmospheres but the K-impact on the compounds production was intensified under oxidative conditions. The furfuryl alcohol yield increased by a factor of 30 for Amapaí comparing higher K% and demineralized samples, whereas a factor of 3 was observed for inert conditions. For Miscanthus, the factors of increasing production were 22 and 5 for oxidative and inert atmosphere. Under oxygen-lean, syringaldehyde and vanillin yields decreased with higher K%, whereas inert treatment revealed insignificant effects. The results indicate that torrefaction duration and targeted condensates production could be improved according to the K% and atmosphere.