We report a new application of Forward Degenerate Four Wave Mixing to
the two-photon transition 2p --> 3p of atomic oxygen. This spectroscop
y has been used to detect oxygen atoms produced in a microwave dischar
ge. The possibility to determine number densities over a range of micr
owave plasma parameters is investigated. In contrast to conventional l
inear techniques, quantitative data are relatively easy to obtain even
for environments with high background luminosity, limited optical acc
ess and inhomogeneous quenching.