The density of neutral oxygen atoms in a plasma postglow chamber was measured with a standard Ni catalytic probe and a fiber optical catalytic probe. A highly dissociated oxygen plasma was created in the discharge chamber with an inductively coupled rf generator with a frequency of 27.12 MHz and output power of 200 W. The measurements were performed in the postglow chamber where a movable recombinator for oxygen atoms was placed. The recombinator enabled the adjustment of O density independently of discharge parameters. The density of neutral oxygen atoms was determined at different pressures and different positions of the recombinator. The O density was between 5 x 10(20) and 2 x 10(21) m(-3). The fiber probe expressed excellent repeatability as compared to the standard probe. As long as the temperature of the catalyst was above 100 degreesC both probes gave similar results. At a lower temperature of the standard probe, however, the optical fiber probe gave more reasonable results. The discrepancy was attributed to the decrease of the catalytic activity of nickel at lower temperatures.