A comparative study of two different absolute methods NO titration and fiber optics catalytic probe (FOCP) for determination of neutral oxygen atom density is presented. Both methods were simultaneously applied for measurements of 0 density in a postglow of an Ar/O-2 plasma created by a surfatron microwave generator with the frequency of 2.45 GHz an adjustable output power between 30 and 160 W. It was found that the two methods gave similar results. The advantages of FOCP were found to be as follows: it is a nondestructive method, it enables real time measuring of the 0 density, it does not require any toxic gas, and it is much faster than NO titration. The advantage of NO titration was found to be the ability to measure 0 density in a large range of dissociation of oxygen molecules. (C) 2003 American Vacuum Society.