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We present the second part of our detailed analysis of the hot sdO andspectroscopic standard star BD+28 4211, in which we focus on the opticalspectrum. In the first part of our study, we determined the abundances of some11 metals detected in the atmosphere of BD+28 4211 using UV spectra of the starand corroborated the fundamental parameters estimated in past studies (Teff$\sim$ 82,000 K, log g $\sim$ 6.2, and solar N(He)/N(H)). In this work, we aimat rederiving these secured parameters on the sole basis of high-qualityoptical spectra. A first grid of non-LTE line-blanketed model atmospheres,including metals with the abundances derived from the UV spectrum, does notgive satisfactory results when we apply a standard simultaneous fittingprocedure to the observed H and He lines of our optical spectra. The lineprofiles are not finely reproduced and the resulting effective temperatures, inparticular, are too low by $\sim$10,000 K. We next investigate the probablecause of this failure, that is, the importance of missing opacity sources onthe atmospheric stratification. We compare line profiles computed from modelswith artificially boosted metallicities, from solar abundances to 15$\times$these values. We find that the structural effects saturate for a metallicity of$\sim$10x solar, and use this to compute a second full grid of models andsynthetic spectra. This metal-enriched grid allows us to achieve significantlyimproved spectral fits with models having the expected parameters. Our testcase thus reveals that there is still a need for models with enhancedmetallicity for better estimating the atmospheric parameters of objects such ashot subdwarfs and hot white dwarfs if only optical spectra are available.

A NLTE analysis of the hot subdwarf O star BD+28 4211. II. The optical spectrum