Aerosol hygroscopicity is a key property governing aerosol water uptake and aerosol–cloud interactions, yet the hygroscopicity of organic aerosol (OA) remains poorly constrained because of its chemical complexity and diverse emission sources. Here, OA hygroscopicity (<em>κ</em><sub>OA</sub>) was retrieved from comprehensive field observations at a rural site in eastern China using humidified light-scattering measurements and ZSR-based closure analysis. <em>κ</em><sub>OA</sub> exhibited pronounced temporal variability during winter, ranging from nearly zero to 0.49 with a mean value of 0.11 ± 0.11. Although OA was generally highly oxidized, <em>κ</em><sub>OA</sub> was only weakly correlated with the O:C ratio, indicating that bulk oxidation state alone cannot explain its variability. Instead, <em>κ</em><sub>OA</sub> increased systematically with biomass-burning influence. Atmospheric aging substantially enhanced <em>κ</em><sub>OA</sub> under biomass-burning-dominated conditions, whereas highly oxidized OA under weak biomass-burning influence remained weakly hygroscopic. These findings demonstrate that the hygroscopic response of OA to atmospheric aging is fundamentally source dependent and indicate that source-dependent parameterizations provide a more physically realistic framework than conventional oxidation-based approaches for representing OA hygroscopicity in atmospheric models.