The temporal and spatial evolution of oxygen spectral line intensities has been measured in the STP-2 screw-pinch tokamak. The electron density and temperature as measured by Thomson scattering are of the order of 1015 cm−3 and 10 eV, respectively. On the basis of these measurements, quasi-steady-state rate equations have been solved to give the OII and OIII ion densities from measurements of oxygen line intensities.It is found that the density of the oxygen impurity is about several percent of the electron density, and the impurity moves with the bulk plasma. It is confirmed that the impurity originates from the wall of the discharge tube during the initial phase of the discharge.