  We present the second part of our detailed analysis of the hot sdO and
spectroscopic standard star BD+28 4211, in which we focus on the optical
spectrum. In the first part of our study, we determined the abundances of some
11 metals detected in the atmosphere of BD+28 4211 using UV spectra of the star
and 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 aim
at rederiving these secured parameters on the sole basis of high-quality
optical spectra. A first grid of non-LTE line-blanketed model atmospheres,
including metals with the abundances derived from the UV spectrum, does not
give satisfactory results when we apply a standard simultaneous fitting
procedure to the observed H and He lines of our optical spectra. The line
profiles are not finely reproduced and the resulting effective temperatures, in
particular, are too low by $\sim$10,000 K. We next investigate the probable
cause of this failure, that is, the importance of missing opacity sources on
the atmospheric stratification. We compare line profiles computed from models
with artificially boosted metallicities, from solar abundances to 15$\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 and
synthetic spectra. This metal-enriched grid allows us to achieve significantly
improved spectral fits with models having the expected parameters. Our test
case thus reveals that there is still a need for models with enhanced
metallicity for better estimating the atmospheric parameters of objects such as
hot subdwarfs and hot white dwarfs if only optical spectra are available.
