The downstream zone of a new type of plasma source was investigated. Microwave power is fed to the plasma by an annular waveguide ring with slotted Line radiators on the inner side. In a pure oxygen plasma the conditions for the generation of atomic oxygen were studied by laser-induced fluorescence. Excitation from the ground state is achieved by two-photon absorption (3p P-3(1,2,0)<--2p P-3(2,1,0) transitions) of 226 nm laser radiation. Fluorescence is observed at 845 nm. From the Doppler-broadened absorption line profile a temperature of 350 K is determined. A rate equation model combined with a measurement of laser properties and a calibration of the detection system by Raman scattering in hydrogen gas enabled an absolute calculation of the atomic oxygen ground state density. In the downstream zone values in the range of 10(13)-10(14) cm(-3) are obtained. The scaling of the atomic oxygen density with external parameters was investigated.