The spectroscopy of the 2p4 3P-2p33s' 3D0(989 ¿) transition of atomic oxygen and the intermediate branch transition in 2p33p 3P-2p33s' 3D0(7990 ¿) has been modelled for an optically thick thermosphere using Monte Carlo simulation. The model reproduces rocket observations of 989 ¿ in the dayglow and aurora if the e+O electron impact excitation cross section is 8.4¿10-18 cm2 at 25 eV. The individual 989-¿, multiplet strengths agree with dayglow observations which partially resolve the line structure. The 7990-¿ singlet to doublet ratio observed in auroras is also consistent with the model. No evidence is found for selective absorption of individual triplet and doublet lines of the 989-¿ transition through accidental resonances with rotational lines of N2. This is compatible with the latest wavelength data which indicate that O I and N2 lines are off resonance by two Doppler widths. The 7990/989 intensity ratio in an aurora is sensitive to the abundance of atomic oxygen and the height distribution of the initial excitation rate and, to a lesser extent, to absorption by O2. Ratios of the 7990-¿ multiplets primarily depend on the O abundance with second-order dependence on O2 absorption and the characteristic energy of the aurora. Accurate ground-based observations of the 7990-¿ multiplet ratios may yield the atomic oxygen abundance in auroras to within a factor of 2.