Two experimental methods have been used to quantify the atomic oxygen density of the hot air plasma generated by a microwave resonant cavity (2.45GHz). The first method is the standard two photon absorption laser induced fluorescence (TALIF) using xenon for calibration but applied for the first time in such post discharge hot air plasma with temperature up to about 4500K. The second method is a kind of actinometry based on the comparison of the emission lines in the visible range of specific atomic oxygen line (844nm) and xenon line (823nm) coming from the addition of a controlled low proportion of a chemically inert gas (1%Xe) in the air plasma.