Chemical oxygen generators (COGs) have limited use in underground emergency refuge spaces due to their relatively high oxygen generation rate (OGR) and high thermal output. We evaluated the effects of catalyst, metal fuel, and block structure on the OGR and surface combustion temperature of several formulations of COGs through thermogravimetric analysis (TGA) and COG block experiments. A bilayer-structured COG with a bottom layer consisting of 88% sodium chlorate, 4% manganese, 5% cobalt(II, III) oxide, and 3% kaolin, and a top layer consisting of 92% sodium chlorate, 5% cobalt(II, III) oxide, and 3% kaolin, at a mass ratio of 1:4, was found to be optimal. The mean OGR of the optimized COG was almost half that of the best single-layered sample.