  The ion populations most frequently adopted for diagnostics in collisional
plasmas are derived from the density independent, coronal approximation. In
higher density, lower temperature conditions, ionisation rates are enhanced
once metastable levels become populated, and recombination rates are suppressed
if ions recombine into Rydberg levels. As a result, the formation temperatures
of ions shift, altering the diagnostics of the plasma. To accurately model the
effect of ionisation from metastable levels, new electron impact, ionisation
cross sections have been calculated for oxygen, both for direct ionisation and
excitation--auto-ionisation of the ground and metastable levels. The results
have been incorporated into collisional radiative modelling to show how the
ionisation equilibrium of oxygen changes once metastable levels become
populated. Suppression of dielectronic recombination has been estimated and
also included in the modelling, demonstrating the shifts with density in
comparison to the coronal approximation. The final results for the ionisation
equilibrium are used in differential emission measure modelling to predict line
intensities for many lines emitted by O II-VI in the solar transition region.
The predictions show improved agreement by 15-40% for O II, O VI and the
inter-combination lines of O III-V, when compared to results from coronal
approximation modelling. While there are still discrepancies with observations
of these lines, this could, to a large part, be explained by variability in the
observations.
