Organic aerosol (OA) is a major component of PM_2.5 with significant health impacts, particularly in the highly populated NCP. However, characterizing OA composition, toxicity, and sources remains challenging. To address this, we conducted an intensive campaign throughout the 2023–2024 heating season in Weifang, a representative medium-sized NCP city. Despite overall air quality improvements, severe winter PM_2.5 pollution events still occurred, with a maximum concentration reaching 985 µg m^-3. Using a Chemical Ionization Time-of-Flight Mass Spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS), we obtained molecular composition and identified six clusters, linking them to sources resolved by positive matrix factorization (PMF) applied to the online dataset, including secondary inorganics, biomass burning, vehicle emissions, coal combustion, and dust. To evaluate toxicity, we determined the oxidative potential (OP) in Weifang. While volume-normalized OP (OP_v) increased with both the degree of unsaturation and O:C ratios of OA, the compounds most positively correlated with mass-normalized OP (OP_m) exhibited an average carbon oxidation state of -0.5 and O:C of 0.5, indicating that moderately oxygenated OA possesses high OP activity. Multiple linear regression revealed that two combustion-derived groups drive OP: highly unsaturated C_10-15 compounds with ~10 oxygen atoms from solid fuel burning, and C_>15 compounds with <5 oxygen atoms or nitrogen-containing compounds from traffic emissions, which exhibited OP_m up to 23 times higher than the ambient average. Our study highlights the continued need for emission controls, particularly those targeting solid fuel combustion in the NCP region beyond its megacities.