The upstream applications of post-consumer polypropylene (PCPP) demand long-term oxidation stability to prevent deterioration of its properties. This study explores the potential to replace a commercial antioxidant (AO), Irganox 1010 (I1010), with greener alternatives, quercetin and tocopherol. The emphasis was placed on evaluating their influence on thermal stability, including oxidation resistance and degradation behavior during multiple processing cycles. The rPCPP exhibited inferior thermal properties, with an oxidation induction time (OIT) of 1.2 min and an oxidation onset temperature (OOT) of 205°C. Incorporation of the selected individual AOs improved these properties. The combination of quercetin and tocopherol (QT) exhibited the best results, increasing OIT to 43 min and OOT to 260°C. After five reprocessing cycles, the melt flow rate (MFR) values for the QT system remained unchanged, whereas I1010-stabilized and the non-stabilized rPCPP increased by 183% and 376%, respectively. Similarly, the carbonyl index (CI) value increased by four times for rPCPP and two times for I1010, whereas the QT remained nearly unchanged. The observed enhanced stability of QT is attributed to the synergistic effect. This effect was further supported by the DPPH assay. All the findings demonstrate the effectiveness of selected natural AOs, particularly QT, as a sustainable alternative to conventional stabilizers.