TiO2- and SiO2-supported Co3O4, Pt and Co3O4-Pt catalysts have been studied for CO and C3H8 oxidations at temperatures less than 250℃ which is a lower limit of light-off temperatures to oxidize them during emission test cycles of gasoline-fueled automotives with TWCs (three-way catalytic converters) consisting mainly of Pt, Pd and Rh. All the catalysts after appropriate activation such as calcination at 350℃ and reduction at 400℃ exhibited significant dependence on both their preparation techniques and supports upon CO oxidation at chosen temperatures. A Pt/TiO2 catalyst prepared by using an ion-exchange method (IE) has much better activity for such CO oxidation because of smaller Pt nanoparticles, compared to a supported Pt obtained via an incipient wetness (IW). Supported Co3O4-only catalysts are very active for CO oxidation even at 100℃, but the use of TiO2 as a support and the IW technique give the best performances. These effects on supports and preparation methods were indicated for Co3O4-Pt catalysts. Based on activity profiles of CO oxidation at 100℃ over a physical mixture of supported Pt and Co3O4 after activation under different conditions, and typical light-off temperatures of CO and unburned hydrocarbons in common TWCs as tested for C3H8 oxidation at 250℃ with a Pt-exchanged SiO2 catalyst, this study may offer an useful approach to substitute Co3O4 for a part of platinum group metals, particularly Pt, thereby lowering the usage of the precious metals.