Reliable quantification of organic carbon (OC) is essential for the comparability of long-term aerosol measurements across monitoring networks. A multi-site intercomparison of OC measurements was conducted at three Central European background stations, Melpitz (MEL), Frýdlant (FRY), and Košetice (NAOK), using aerosol mass spectrometry (AMS/ACSM) and thermal-optical transmission (TOT) methods (offline and semi-continuous). Semi-continuous and offline TOT measurements showed high consistency across sites and seasons, suggesting negligible influence of particle size differences on OC comparability. A pronounced seasonal contrast was observed between AMS derived OC (OC_AMS) and TOT based OC (OC_TOT). During summer, good agreement was found with OC_TOT:OC_AMS slopes of 1.27 (MEL), 0.86 (FRY), and 0.83 (NAOK). In winter, AMS systematically underestimated OC, with slopes decreasing to 0.34 (MEL), 0.24 (FRY), and 0.63 (NAOK). The discrepancy increased under combustion dominated conditions and was associated with low organic aerosol to equivalent black carbon (OA:eBC) ratios (<≈5), identifying a regime where OC_AMS and OC_TOT diverge substantially. Source dependent effects were evident, with the largest deviations observed at the coal influenced site (FRY) and during long-range transport events, while improved agreement coincided with higher OA:eBC ratios and enhanced secondary organic aerosol formation in summer. Evaluation of potential uncertainties (including pyrolytic carbon formation in TOT and AMS related limitations such as refractory organics and ionization efficiency) showed that no single factor explains the winter discrepancy, suggesting combined matrix and source effects. The results show that OA:eBC can serve as a practical indicator for OC_AMS-OC_TOT agreement and provide recommendations to improve OC quantification under winter combustion conditions.