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We have analyzed the IUE spectra which covers the early optically thick phase of Nova OS And 1986. We use spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres to calculate a grid of co-moving frame, NLTE atmospheres for this nova with the following set of parameters: L=2 $\times$ 10$^{5}$ L$_\odot$, $\rho \propto$ r$^{-3}$, statistical velocity = 50 km s$^{-1}$, 5000 $\le T_{\rm eff} \le 60000\,$K, $v_{\rm max}$ = 2000 km s$^{-1}$ and solar abundances. We then computed observer frame synthetic spectra for each of the model atmospheres in the grid and used them to estimate the model parameters corresponding to the observed IUE spectra. The fit to the observations was iteratively improved by changing the parameters of the model atmospheres, in particular $T_{\rm eff}$, $v_{\rm max}$, and the abundances.
The synthetic spectra give a very good fit to the pseudo-continuum and many of the features found in the IUE spectra. The earliest spectra, approximately 4 and 5 days after maximum light in the optical, are best fit by models with solar abundances for all elements except for carbon and oxygen which have abundances enhanced by a factor of about ten. We can not determine nitrogen abundances due to the lack of strong nitrogen lines in the ultraviolet. The later spectra, taken 19 and 27 days after maximum light, show, simultaneously, allowed, semi-forbidden, and forbidden lines. For these epochs, we can fit the allowed lines and the pseudo-continuum with solar abundances, except for carbon and oxygen which are still required to be enhanced by a about a factor of ten. The semi-forbidden and forbidden lines are formed in the emerging pre-nebula and are not$, currently, included in the model atmospheres. We also find evidence for large non-LTE effects of Fe II in the later observations.
