Oxygen (O-2) and carbon monoxide (CO) are produced in combustion reactions and are important gas species, which are closely related to combustion efficiency and production of air pollutants. O-2 generates thermal nitrogen oxide (NOx) by reacting with nitrogen in the air at high temperatures, and the presence of excess O-2 also affects the energy efficiency as a result of increasing exhaust heat energy. CO is generally well-known for being a toxic gas and is a crucial gas species produced by incomplete combustion. However, it is difficult to measure O-2 and CO level variations in a huge combustion system, such as used in steel annealing and power plant boilers, because of their harsh environment. Therefore, as a multi-species measurement technique, which is non-intrusive and has a high sensitivity and high response, tunable diode laser absorption spectroscopy was chosen as the optical method used to measure O-2 and CO concentrations in exhaust gases. In this study, experiments were carried out to measure the O-2 and CO concentrations in an electrical furnace and a combustion system using direct absorption spectroscopy and wavelength modulation spectroscopy. The measured results were compared to those of a gas analyzer of the electrochemical sensor type. The O-2 and CO concentrations were measured in the exhaust gas produced from methane/air flame through adjustment of the equivalence ratio to form both fuel-lean and fuel-rich conditions. To measure the O-2 and CO concentrations precisely and without interference from other combustion products, visible and near-infrared wavelength regions at 760.8 and 2325.2 nm were selected, respectively.