In this study we investigate the hypothesis that oxidants present within atmospheric particles can promote the formation of highly oxidized organic aerosol (OA) via oxidation reactions in the condensed phase. Secondary organic aerosol (SOA) was generated from the ozonolysis of α-pinene and isoprene in an environmental chamber, with seed particles systematically varied in order to assess the effects of condensed-phase oxidant levels on SOA loading and composition. The effects of particle phase (aqueous vs dry), condensed-phase oxidant source (none vs H2O2 vs Fenton chemistry), and irradiation (none vs UV) were all examined. For experiments conducted with aqueous particles but without any added oxidants, UV irradiation resulted in a small but measurable enhancement in the oxygen-to-carbon ratio (O/C). OA formed in the presence of aqueous oxidants was substantially more oxidized, with the highest oxidant concentrations leading to OA with an O/C as high as 1.4 for α-pinene and 2.0 for isoprene, strongly suggesting the formation of oxalate. High aqueous oxidant levels also resulted in increased loss of carbon from the condensed phase. This OA was more oxidized than in any other ozonolysis experiment reported to date, indicating that, when present, aqueous oxidants can have a dramatic effect on SOA formation. However, oxidant concentrations within atmospheric aqueous particles remain poorly constrained, making it difficult to assess the impacts of aqueous-phase oxidation on the loadings and oxidation state of atmospheric OA.