We report a theoretical and experimental investigation of atomic oxygen detection in an atmospheric-pressure flame by two-photon degenerate four-wave mixing (TPDFWM). We present analytical expressions for the TPDFWM signal that are based on the diagrammatic perturbation theory. The description of several polarization arrangements is given and used in our experiments. The importance of the polarization scheme is emphasized and the most efficient scheme is found by investigating signal and noise contributions. In this case, thermal grating contributions are greatly reduced and this allows quantitative concentration measurements in flame to be performed using TPDFWM.