Abstract The paper presents a calibration of the ratio of two-photon absorption cross-sections, σ X e ( 2 ) / σ O ( 2 ) , necessary for the absolute O-atom density measurements by two-photon absorption laser-induced fluorescence (TALIF) technique. To calibrate the ratio of the cross-sections, a special discharge with 100% dissociation of molecular oxygen, and so with a known ‘reference’ density of O-atom [O] r e f = 2 ⋅ [O 2 ] was suggested. This is a nanosecond capillary discharge in N 2 :O 2 mixtures with a few percent of oxygen at a reduced electric field of a few hundred of Townsend and specific deposited energy of about 1 eV mol −1 . Voltage at the electrodes, electrical current in the plasma, longitudinal electric field and energy delivered to the gas were measured with 0.2 ns synchronisation. Additionally, radial distribution of emission of excited nitrogen molecules and gas temperature in the discharge and afterglow were obtained experimentally. Detailed 1D kinetic modeling was suggested to confirm complete O 2 dissociation and to analyse the main reactions. By comparing the data measured by TALIF technique with the ‘reference’ density of oxygen atoms [O] r e f , the ratio of the two-photon absorption cross-sections σ X e ( 2 ) / σ O ( 2 ) was determined.