The gas-phase oxidation of 2,5-dimethylfuran (2,5-DMF) by ozone (O₃) and hydroxyl radicals (OH) was investigated in flow reactors at atmospheric pressure and room temperature using nitrate chemical ionization orbitrap mass spectrometry. At a residence time of 0.8 s, highly oxygenated organic molecules (HOM) with up to nine oxygen atoms were detected in the ozonolysis channel, and eight oxygen atoms in the OH channel. These observations demonstrate that sequential intramolecular hydrogen-shift autoxidation is sufficiently rapid to form products with up to nine oxygen atoms on sub-second timescales. At 7 s, C₁₀–C₁₂ accretion products form through different combinations of C₅ and C₆ alkyl peroxy (RO₂) radicals. Quantum chemical calculations using density functional theory and coupled-cluster methods identified a distinct Criegee intermediate geometry (Anti-CI-2A) that provides a plausible route to HOM species containing up to nine oxygen atoms. Its different methyl/hydrogen orientation relative to the conventional Syn and Anti conformers enables a rapid 1,6-H shift, facilitating autoxidation toward O₉ formation and suggesting that primary ozonide decomposition in structurally complex ozonolysis systems may access reactive Criegee intermediate geometries beyond the conventional Syn and Anti forms. For OH-initiated oxidation, the proposed mechanism accounts for HOM monomer formation up to O₆, with rapid termination and radical recycling limiting further autoxidation. Overall, 2,5-DMF produces low-volatility oxidation products from both ozonolysis and OH-initiated pathways with implications for secondary organic aerosol formation.