Fermentation concentrate of Hericium caput-medusae (HFC) was used in this study and two different molecular weight oligosaccharides (HFCO1, HFCO2) were obtained by dialysis methods. Furthermore, DEAE Sepharose Fast Flow and Sephadex G-25 column chromatography were used to obtain the two purified components (HFCO1-1, HFCO2-1). The two components were partially characterized, and analysis of monosaccharide composition through Fourier-transform infrared spectroscopy was performed. The sugar-containing components in HFC were mainly neutral oligosaccharides, and the monosaccharide composition was mainly glucose, with the configuration of α-pyranose. Their chemical and cellular antioxidant activities were compared, and the results showed that antioxidant activities of HFCO2 were higher than HFCO2-1 and antioxidant activities of crude oligosaccharides were higher than those of neutral oligosaccharides. By both in vitro and in vivo studies of scavenging activities toward the free radicals and increasing the activity of antioxidant enzymes in cells, HFCO2 and HFCO2-1 could suppress H2O2-induced reduction in cell viability.