Linear and nonlinear absorption of the overtone CO laser radiation by different atmospheric gases is considered. Emission lines are selected for using in spectroscopic absorption diagnostics of particular molecules. Emission and absorption spectral characteristics of the overtone CO laser are analyzed in comparison with the chemical НF and DF lasers. The information on the 50 most weakly and 50 most strongly absorbed in the atmosphere lines of the overtone CO laser is presented. The absorption is measured at different frequencies of the overtone CO laser in the N2O/N2 mixture. It is shown that using the dependence of N2O absorption coefficient on the foreign gas pressure it is possible to identify spectral lines of the overtone CO laser and determine its frequency. Nonlinear absorption spectra are calculated for high-intensity multifrequency and monochromatic radiation of the overtone CO laser in the atmosphere to demonstrate the feasibility of using this laser in new promising schemes of diagnostics.