Measurements at 298 K of the intensities of atomic resonance absorption and of atomic resonance fluorescence, as functions of ground state O 3PJ oxygen atom concentrations, have been made for each of the three transitions (near λ 130 nm) of the O 3S1–3P2,1,0 multiplet. The light source was an oxygen atom lamp in argon carrier gas, and O 3PJ oxygen atoms were generated in a discharge–flow system, both at low pressure (< 200 N m–2). Simple theory shows that the resonance absorption intensity, A=Iabs/I0, varies with optical depth in the absorber, k0l, (k0l∝[O 3PJ]), according to a polynomial, A=(k0l)α–½(k0l)2β+…, always leading to the limiting case A→(k0l)α as k0l→0. Several models for the line profile of the atomic resonance lamp are used to compute values for A and the coefficients α, β…, for comparison with the experimental values obtained for these parameters. The results are extended to include the case of Lorentz-broadened absorption lines, in order to relate absorption measurements in low-pressure flow systems to those in higher pressure experiments (e.g. pulsed photolysis studies). The intensity of resonance fluorescence IF at low pressure is considered as a function of the intensity of absorption from the source, and a fluorescence emission function. The effective absorption coefficient for O 3PJ atoms is much higher for the non-reversed fluorescent radiation emitted homogeneously in the fluorescence cell, than for the radiation originating from a typical (self-reversed) source. Consequently, the concentration of O 3PJ atoms can be low enough to be optically thin with respect to absorption of radiation from the source but high enough to be optically thick with respect to re-absorption of fluorescence emitted within the homogeneous mixture of emitters and absorbers in the fluorescence cell. These considerations are used to obtain an approximate model for the variation of IF with [O 3PJ]—as for absorption, the model leads to a polynomial in k0l with the same experimentally-observed limiting case IF→C′(k0l) as k0l→0. The results show that whilst the O 3S1–3P2(λ 130.2 nm) line is more sensitive than the O 3S1–3P0(λ 130.6 nm) line for the detection of O 3PJ atoms, the range of linear dependence of IF upon total oxygen atom concentration extends to much higher concentrations for λ 130.6 than for the λ 130.2 transition.